rename hints/include/multiw-3.370 to hints/include/misc.370
keep the portable nroff options in sys/unix/Makefile.doc,
and relocate the non-portable bits to a variable defined
in sys/unix/hints/include/misc.370
This assumes that the groff options are compatible between
Linux and macOS implementations of groff.
If that turns out not to be the case, this bit:
ifneq "$(NROFFISGROFF)" "" # It's groff
# add the groff-specific plain text flags
MORE_MAN2TXT_FLAGS += -Tascii -P -cbou
endif
should relocate from sys/unix/hints/include/misc.370
to sys/unix/hints/linux.370 and sys/unix/hints/macOS.370,
immediately following the inclusion of misc.370, and the
appropriate platform-specific groff options can be
adjused in whichever of those appropriately needs it.
Closes#1153
