Partial support for the \n[an-margin] number register.

Partial support for the \n[an-margin] number register.

Manuals autogenerated from reStructuredText are reckless enough
to peek at this non-portable, implementation-dependent, highly
groff-specific internal register - for no good reason, because the
man(7) language natively provides in a much simpler way what they
are trying to emulate here with much fragility.

A full implementation would be very hard because it would require
access to output-device-specific formatting data at the roff(7)
preprocessor stage, which mandoc doesn't support at all.
So hardcode a few magic numbers as reStructuredText expects them
for terminal output.  For other output modes (like HTML), code using
this register is utterly broken anyway.