Mostly complete implementation of the 'c' (character available)

Mostly complete implementation of the 'c' (character available)
roff conditional, except that the .char request still isn't supported
and that behaviour differs from groff in many edge cases.
But at least valid character names and numbers are now distinguished
from invalid ones.
This also fixes the bug that parsing of the 'c' conditional was
incomplete, which resulted in leaking the tested character to the
input parser at the beginning of the body when the condition was
inverted.