op_get: support getting argument in next argv item

op_get: support getting argument in next argv item

previously arguments needed to be passed with an equals sign like so:
--foo=bar, now also --foo bar is supported.

note that from a static link bloat perspective it's not optimal to add a
dependency on strcmp(), however we do not actually endorse usage of the
optparser anyway, as its NIH design is quite suboptimal and the result of
a lack of understanding of getopt(), at the time it was written.