Guidebook datestamping

Allow the hints file to apply a correct timestamp to
the Guidebooks prior to use.

Detect the NH_DATESUB in the Guidebook.mn or Guidebook.tex
files and replace the datestamping line that follows.

If git is available, it determines the hash of the last commit
applied to doc/Guidebook.mn, and then determines the date of
that commit.  The interim Guidebook.dated.mn (or
Guidebook.dated.tex) gets the datestamp applied.

If git isn't available or doesn't correctly provide the hash
for doc/Guidebook.mn, it should just continue to use whatever
hard-coded date in the Guidebook.mn source file (it uses it
instead of the interim file).

sys/unix/hints/linux.370

@@ -336,7 +336,17 @@ GAMEPERM = 0755
 #
 #-INCLUDE cross-pre.370
 #
+
+#
+#-INCLUDE gbdates-pre.370
+#
+
 #-POST
+
+#
+#-INCLUDE gbdates-post.370
+#
+
 #
 #-INCLUDE cross-post.370
 #