card: New commands "gpg" and "gpgsm".

* tools/gpg-card.c: Include exechelp.h
(cmd_gpg): New.
(enum cmdids): Add cmdGPG and cmdGPGSM.
(cmds): Add commands "gpg" and "gpgsm"
(dispatch_command, interactive_loop): Call them.

It is too cumbersome to leave the gpg-card shell just for running a
quick gpg or gpgsm command. Thus we add these new commands.

Take care: As of now we don't have proper shell-quoting rules
implemented. This will eventually be done.