Centralise stripslashes() and make it OS-sensitive.

I noticed that Unix PSCP was unwantedly renaming downloaded files
which had a backslash in their names, because pscp.c's stripslashes()
treated \ as a path component separator, since it hadn't been modified
since PSCP ran on Windows only.

It also turns out that pscp.c, psftp.c and winsftp.c all had a
stripslashes(), and they didn't all have quite the same prototype. So
now there's one in winsftp.c and one in uxsftp.c, with appropriate
OS-dependent behaviour, and the ones in pscp.c and psftp.c are gone.

psftp.h

@@ -177,4 +177,21 @@ int create_directory(const char *name);
  */
 char *dir_file_cat(const char *dir, const char *file);
 
+/*
+ * Return a pointer to the portion of str that comes after the last
+ * path component separator.
+ *
+ * If 'local' is false, path component separators are taken to just be
+ * '/', on the assumption that we're discussing the path syntax on the
+ * server. But if 'local' is true, the separators are whatever the
+ * local OS will treat that way - so that includes '\' and ':' on
+ * Windows.
+ *
+ * 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 :-(
+ */
+char *stripslashes(const char *str, int local);
+
 #endif /* PUTTY_PSFTP_H */