Implement the roff(7) .rn (rename macro or string) request.

Implement the roff(7) .rn (rename macro or string) request.
Renaming a user-defined macro is very simple: just copy
the definition to the new name and delete the old name.
Renaming high-level macros is a bit tricky: use a dedicated
key-value-table, with non-standard names as keys and standard
names as values.  When a macro is found that is not user-defined,
look it up in the "renamed" table and translate it back to the
standard name before passing it on to the high-level parsers.