upstream: switch sshd_config parsing to argv_split()

similar to the previous commit, this switches sshd_config parsing to
the newer tokeniser. Config parsing will be a little stricter wrt
quote correctness and directives appearing without arguments.

feedback and ok markus@

tested in snaps for the last five or so days - thanks Theo and those who
caught bugs

OpenBSD-Commit-ID: 9c4305631d20c2d194661504ce11e1f68b20d93e
