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.

unix/uxsftp.c

@@ -413,6 +413,20 @@ void finish_wildcard_matching(WildcardMatcher *dir) {
     sfree(dir);
 }
 
+char *stripslashes(const char *str, int local)
+{
+    char *p;
+
+    /*
+     * On Unix, we do the same thing regardless of the 'local'
+     * parameter.
+     */
+    p = strrchr(str, '/');
+    if (p) str = p+1;
+
+    return (char *)str;
+}
+
 int vet_filename(const char *name)
 {
     if (strchr(name, '/'))