Cleanups to complete the man page backend. Also, an additional new

Cleanups to complete the man page backend. Also, an additional new
markup feature: a \c line in a code paragraph can now be followed by
an optional \e line indicating emphasised bits of its preceding \c.
This allows discretionary bolding and (italic/underline) emphasis
within code paragraphs, but without introducing an escape character
or breaking any existing input files. Users are warned that not all
backends are required to actually render these hints, and so they
should avoid depending on them 100% to convey semantic information
unless they know they're writing for a restricted range of backends.

git-svn-id: svn://svn.tartarus.org/sgt/halibut@3965 cda61777-01e9-0310-a592-d414129be87e