Don't exit(1) after printing PGP key fingerprints.

That's not a failure outcome. The user asked for some information; we
printed it; nothing went wrong. Mission successful, so exit(0)!

I noticed this because it was sitting right next to some of the
usage() calls modified in the previous commit. Those also had the
misfeature of exiting with failure after successfully printing the
help, possibly due to confusion arising from the way that usage() was
_sometimes_ printed on error as well. But pgp_fingerprints() has no
such excuse. That one's just silly.

psftp.c

@@ -2839,7 +2839,7 @@ int psftp_main(CmdlineArgList *arglist)
             cleanup_exit(0);
         } else if (strcmp(argstr, "-pgpfp") == 0) {
             pgp_fingerprints();
-            return 1;
+            return 0;
         } else if (strcmp(argstr, "-V") == 0 ||
                    strcmp(argstr, "--version") == 0) {
             version();