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/hints/linux.370

@@ -29,7 +29,7 @@ PRECHECK+=checkmakefiles
 # in HINTSINCLNAMES (without suffix and without a path)
 HINTSINCLNAMES := compiler cross-pre cross-post \
 		gbdates-pre gbdates-post \
-		multiw-1 multiw-2 multiw-3 \
+		multiw-1 multiw-2 misc \
 		multisnd1-pre multisnd2-pre multisnd-post
 HINTSINCLFILES := $(addsuffix .$(HINTSVERSION), $(HINTSINCLNAMES))
 endif
@@ -65,10 +65,10 @@ QTDIR=/usr/local/qt6
 endif   # WANT_WIN_QT6
 endif  # WANT_WIN_QT
 
-# multiw-3.370 must come after compiler.370
+# misc.370 must come after compiler.370
 # and after QTDIR is defined.
 #
-#-INCLUDE multiw-3.370
+#-INCLUDE misc.370
 
 ifeq "$(USE_ASAN)" "1"
 CFLAGS+=-fsanitize=address