Unify mdoc_deroff() and man_deroff() into a common function deroff().

Unify mdoc_deroff() and man_deroff() into a common function deroff().
No functional change except that for mdoc(7), it now skips leading
escape sequences just like it already did for man(7).
Escape sequences rarely occur in mdoc(7) code and if they do,
skipping them is an improvement in this context.
Minus 30 lines of code.