Windows PSCP now links against winsftp.c, and scp.c is now a

platform-independent source file. Haven't yet added the extra
abstraction routines to uxsftp.c to create a Unix PSCP port, but it
shouldn't take long.
Also in this checkin, a change of semantics in platform_default_s():
now strings returned from it are expected to be dynamically allocated.

[originally from svn r3420]

putty.h

@@ -845,5 +845,6 @@ Filename filename_from_str(const char *string);
 const char *filename_to_str(const Filename *fn);
 int filename_equal(Filename f1, Filename f2);
 int filename_is_null(Filename fn);
+char *get_username(void);	       /* return value needs freeing */
 
 #endif