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

mandoc: import version 1.14.6

MFC after: 3 weeks

Update mandoc to cvs snapshot 2019-07-23

This snapshot among other things includes a fix for a crash of mandoc with empty
tbl reported by rea@ (his regression test has been incorporated upstream)

MFC after: 3 weeks

Update mandoc to 1.14.5

MFC after: 2 weeks

Import mandoc 1.14.4

Approved by: re (kib)
MFC after: 1 week