mandoc: workaround lack of macro parsing in list -width

GNU tools parse macros in the -width argument of lists. mandoc does not,
so it calculates an excessive width. This often squeezes the text into
a very narrow column, especially in nested lists.

Implement the easy workaround suggested in the TODO list. When there is
only one macro, at the beginning of the -width argument, this fixes the
formatting as well as a complete solution.

Reviewed by: bapt
Relnotes: yes
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D35245