some macOS doc updates

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.

sys/unix/hints/include/misc.370

@@ -65,6 +65,11 @@ CPLUSPLUS_NEEDED = 1
 endif
 endif
 
+ifdef USE_MANDOC
+NROFF = mandoc
+MAN2TXTPRE=
+MAN2TXTPOST=
+else
 #detection of groff
 NROFFISGROFF := $(shell echo `nroff --version | grep "GNU groff version"`)
 #$(info NROFFISGROFF=$(NROFFISGROFF))
@@ -86,6 +91,7 @@ MAN2TXTPRE += -c
 # groff less than 1.23
 endif
 endif  # end groff-specific
+endif  # not USE_MANDOC
 
 #end of hints/include/misc.370
 #------------------------------------------------------------------------------