Giant const-correctness patch of doom!

Having found a lot of unfixed constness issues in recent development,
I thought perhaps it was time to get proactive, so I compiled the
whole codebase with -Wwrite-strings. That turned up a huge load of
const problems, which I've fixed in this commit: the Unix build now
goes cleanly through with -Wwrite-strings, and the Windows build is as
close as I could get it (there are some lingering issues due to
occasional Windows API functions like AcquireCredentialsHandle not
having the right constness).

Notable fallout beyond the purely mechanical changing of types:
 - the stuff saved by cmdline_save_param() is now explicitly
   dupstr()ed, and freed in cmdline_run_saved.
 - I couldn't make both string arguments to cmdline_process_param()
   const, because it intentionally writes to one of them in the case
   where it's the argument to -pw (in the vain hope of being at least
   slightly friendly to 'ps'), so elsewhere I had to temporarily
   dupstr() something for the sake of passing it to that function
 - I had to invent a silly parallel version of const_cmp() so I could
   pass const string literals in to lookup functions.
 - stripslashes() in pscp.c and psftp.c has the annoying strchr nature

psftp.c

@@ -68,7 +68,7 @@ struct sftp_packet *sftp_wait_for_reply(struct sftp_request *req)
  * canonification fails, at least fall back to returning a _valid_
  * pathname (though it may be ugly, eg /home/simon/../foobar).
  */
-char *canonify(char *name)
+char *canonify(const char *name)
 {
     char *fullname, *canonname;
     struct sftp_packet *pktin;
@@ -77,7 +77,7 @@ char *canonify(char *name)
     if (name[0] == '/') {
 	fullname = dupstr(name);
     } else {
-	char *slash;
+	const char *slash;
 	if (pwd[strlen(pwd) - 1] == '/')
 	    slash = "";
 	else
@@ -172,8 +172,13 @@ char *canonify(char *name)
 /*
  * Return a pointer to the portion of str that comes after the last
  * slash (or backslash or colon, if `local' is TRUE).
+ *
+ * This function has the annoying strstr() property of taking a const
+ * char * and returning a char *. You should treat it as if it was a
+ * pair of overloaded functions, one mapping mutable->mutable and the
+ * other const->const :-(
  */
-static char *stripslashes(char *str, int local)
+static char *stripslashes(const char *str, int local)
 {
     char *p;
 
@@ -190,7 +195,7 @@ static char *stripslashes(char *str, int local)
 	if (p) str = p+1;
     }
 
-    return str;
+    return (char *)str;
 }
 
 /*
@@ -1011,7 +1016,8 @@ int sftp_cmd_ls(struct sftp_command *cmd)
     struct fxp_names *names;
     struct fxp_name **ournames;
     int nnames, namesize;
-    char *dir, *cdir, *unwcdir, *wildcard;
+    const char *dir;
+    char *cdir, *unwcdir, *wildcard;
     struct sftp_packet *pktin;
     struct sftp_request *req;
     int i;
@@ -1901,7 +1907,7 @@ static int sftp_cmd_pling(struct sftp_command *cmd)
 static int sftp_cmd_help(struct sftp_command *cmd);
 
 static struct sftp_cmd_lookup {
-    char *name;
+    const char *name;
     /*
      * For help purposes, there are two kinds of command:
      * 
@@ -1915,8 +1921,8 @@ static struct sftp_cmd_lookup {
      *    contains the help that should double up for this command.
      */
     int listed;			       /* do we list this in primary help? */
-    char *shorthelp;
-    char *longhelp;
+    const char *shorthelp;
+    const char *longhelp;
     int (*obey) (struct sftp_command *);
 } sftp_lookup[] = {
     /*
@@ -2139,7 +2145,7 @@ static struct sftp_cmd_lookup {
     }
 };
 
-const struct sftp_cmd_lookup *lookup_command(char *name)
+const struct sftp_cmd_lookup *lookup_command(const char *name)
 {
     int i, j, k, cmp;
 
@@ -2450,7 +2456,7 @@ static int verbose = 0;
 /*
  *  Print an error message and perform a fatal exit.
  */
-void fatalbox(char *fmt, ...)
+void fatalbox(const char *fmt, ...)
 {
     char *str, *str2;
     va_list ap;
@@ -2464,7 +2470,7 @@ void fatalbox(char *fmt, ...)
 
     cleanup_exit(1);
 }
-void modalfatalbox(char *fmt, ...)
+void modalfatalbox(const char *fmt, ...)
 {
     char *str, *str2;
     va_list ap;
@@ -2478,7 +2484,7 @@ void modalfatalbox(char *fmt, ...)
 
     cleanup_exit(1);
 }
-void nonfatal(char *fmt, ...)
+void nonfatal(const char *fmt, ...)
 {
     char *str, *str2;
     va_list ap;
@@ -2490,7 +2496,7 @@ void nonfatal(char *fmt, ...)
     fputs(str2, stderr);
     sfree(str2);
 }
-void connection_fatal(void *frontend, char *fmt, ...)
+void connection_fatal(void *frontend, const char *fmt, ...)
 {
     char *str, *str2;
     va_list ap;
@@ -2866,7 +2872,7 @@ static int psftp_connect(char *userhost, char *user, int portnumber)
     return 0;
 }
 
-void cmdline_error(char *p, ...)
+void cmdline_error(const char *p, ...)
 {
     va_list ap;
     fprintf(stderr, "psftp: ");