Provide a new function roff_req_or_macro() to parse and handle a request

Provide a new function roff_req_or_macro() to parse and handle a request
or macro, including context-dependent error handling inside tbl(7) code
and inside .ce/.rj blocks.  Use it both in the top level roff(7) parser
and inside conditional blocks.

This fixes an assertion failure triggered by ".if 1 .ce" inside tbl(7)
code, found by tb@ using afl(1).

As a side benefit for readability, only one place remains in the
code that calls the main handler functions for the various roff(7)
requests.  This patch also improves column numbers in some error
messages and various comments.