config: parse_argv fix

config: parse_argv fix

md_config_t::parse_argv: fold md_config_t::parse_argv_part2 into
parse_argv. Fix brokenness introduced by the std::string switchover.

OPTION macro: move single-character options out of the OPTION macro and
into config.cc

Fix ceph_argparse_witharg / ceph_argparse_flag uses to include a
trailing (char*)NULL, to ensure that we terminate with a pointer rather
than a 32-bit int.

Signed-off-by: Colin McCabe <colin.mccabe@dreamhost.com>