Step 1 of restructuring: libmandoc.h. Move all compiler-set-specific

Step 1 of restructuring: libmandoc.h.  Move all compiler-set-specific
stuff into libmandoc.h, including old mdoc.h/man.h/roff.h functions now
used by read.c.  The motivation behind this is to tighten the
relationship between the underlying compilers while keeping parse data
hidden from general callers (e.g., main.c).

While here, also move register values from mandoc.h into libmandoc.h as
noted by schwarze@.  See above for explanation.