Make options with dashes show correctly on option error.

Summary: There is a bug in at least Boost 1.54 where options with dashes `-` in them are
not handled correctly. Basically all but the last substring after the last dash
is returned.

See: https://github.com/facebook/hhvm/issues/2864

So our error string when an option had an invalid argument was just plain wrong.
Since we can't fix boost very easily and are not planning to upgrade yet, we
workaround it with this fix.

Fixes: #2864

Reviewed By: @paulbiss

Differential Revision: D1681063

Signature: t1:1681063:1416505169:7664cff3c23b8fdfec995c54897eb276d44f92d6