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

unix/uxucs.c

@@ -57,7 +57,7 @@ int mb_to_wc(int codepage, int flags, const char *mbstr, int mblen,
 }
 
 int wc_to_mb(int codepage, int flags, const wchar_t *wcstr, int wclen,
-	     char *mbstr, int mblen, char *defchr, int *defused,
+	     char *mbstr, int mblen, const char *defchr, int *defused,
 	     struct unicode_data *ucsdata)
 {
     /* FIXME: we should remove the defused param completely... */