Rudimentary implementation of the roff(7) .while request.

Rudimentary implementation of the roff(7) .while request.
Needed for example by groff_hdtbl(7).

There are two limitations:
It does not support nested .while requests yet,
and each .while loop must start and end in the same scope.

The roff_parseln() return codes are now more flexible
and allow OR'ing options.