hints name change and doc/Makefile

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

sys/unix/Makefile.doc

@@ -115,7 +115,8 @@ Guidebook.dat	: Gbk-1pg-pfx.mn Gbk-1pg-sfx.mn $(GUIDEBOOK_MN) tmac.n tmac.nh \
 	$(ONEPAGECMD) > $@
 
 # plain text output
-MAN2TXT = $(NHGREP) | nroff -man -c -Tascii - | $(COLCMD)
+#MAN2TXT = $(NHGREP) | nroff -man $(MORE_MAN2TXT_FLAGS) - | $(COLCMD)
+MAN2TXT = $(NHGREP) | nroff -man $(MORE_MAN2TXT_FLAGS) -
 nethack.txt	: nethack.6
 	cat nethack.6 | $(MAN2TXT) > nethack.txt
 recover.txt	: recover.6