Print warning for arguments not considered an option.

GnuPG requires that options are given before other arguments. This
can sometimes be confusing. We now print a warning if we found an
argument looking alike a long option without being preceded by the
stop option. This is bug#1343.

• common/argparse.h (ARGPARSE_FLAG_STOP_SEEN): New.
• common/argparse.c (arg_parse): Set new flag.
• g10/gpg.c (main): Print the warning.
• agent/gpg-agent.c (main): Ditto.
• dirmngr/dirmngr.c (main): Ditto.
• g13/g13.c (main): Ditto.
• scd/scdaemon.c (main): Ditto.
• sm/gpgsm.c (main): Ditto.
• tools/gpg-connect-agent.c (main): Ditto.
• tools/gpgconf.c (main): Ditto.