Improve formatting of synopsis section

This patch is about sorting the arguments and using proper mdoc(7) macros
to stylize arguments and command modifiers for much better readability.

Further style fixes in other sections within the bhyve manual page are
going to be worked on in upcoming patches.

Reviewed by: rgrimes
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D24526