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

raw.c

@@ -125,8 +125,8 @@ static void raw_sent(Plug plug, int bufsize)
  */
 static const char *raw_init(void *frontend_handle, void **backend_handle,
 			    Conf *conf,
-			    char *host, int port, char **realhost, int nodelay,
-			    int keepalive)
+			    const char *host, int port, char **realhost,
+                            int nodelay, int keepalive)
 {
     static const struct plug_function_table fn_table = {
 	raw_log,
@@ -214,7 +214,7 @@ static void raw_reconfig(void *handle, Conf *conf)
 /*
  * Called to send data down the raw connection.
  */
-static int raw_send(void *handle, char *buf, int len)
+static int raw_send(void *handle, const char *buf, int len)
 {
     Raw raw = (Raw) handle;