Allow arbitrary argument delimiters for \C, like groff does.

Allow arbitrary argument delimiters for \C, like groff does.
The restriction of only allowing ' as the delimiter was introduced
by kristaps@ on 2011/04/09 when he first supported \C.
For most other escape sequences, similar restrictions were relaxed
later on, but for the rarely used \C, it was apparently forgotten.

While here, reject empty character names: they are never valid.