gpgme-tool: Switch to argparse.c for option parsing.

* src/argparse.c, src/argparse.h: New. Taken from current gnupg.
* src/Makefile.am (gpgme_tool_SOURCES): New.
* src/gpgme-tool.c: Remove all argp.h stuff.
(my_strusage): New.
(main): Change to use argparse.

As a GnuPG related tool the user should get the same experience in
option parsing as with gpg et al. Thus we use the same parser.

• Signed-off-by: Werner Koch <wk@gnupg.org>