At least on Debian Linux, after invoking getopt() with argc==0, optind
seems to remain at the default value of 1.

mutt_dotlock was checking for optind==argc to report a missing
parameter, and would end up invoking the function with the first
environment variable instead.

mutt, pgpewrap, and mutt_pgpring were assuming argv[0] was non-null,
so just add a check and hardcode the value in that case to avoid a
possible segv.

mutt_pgpring allows no arguments.  Its parameters to
pgpring_find_candidates() would pass the environment list and a
negative nhints in that case.  The code seem to handle the case, but
set optind=argc to be clear about it.