Allow 'make USE_MANDOC=1 distrib' to be used for generating
the doc/*.txt files for systems that have mandoc and may not have groff.
(macOS Ventura doesn't include groff in the OS).
USE_MANDOC=1 is not restricted only to macOS.
Have hints/macOS.370 specify the /usr/share/man/man6 directory for the macOS
man pages, so that 'make manpages' will target the correct directory on
that platform.
cbeb8a51a3