Add canonical mount options zfs-mount-generator

lib/libzfs/libzfs_mount.c:zfs_add_options provides the canonical
mount options used by a `zfs mount` command. Because we cannot call
`zfs mount` directly from a systemd.mount unit, we mirror that logic
in zfs-mount-generator.

The zed script is updated to cache these properties as well.

Include a mini-tutorial in the manual page, properly substitute
configuration paths in zfs-mount-generator.8.in, and standardize the
Makefile.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Antonio Russo <antonio.e.russo@gmail.com>
Closes #7453

man/man8/Makefile.am

@@ -4,7 +4,6 @@ dist_man_MANS = \
 	vdev_id.8 \
 	zdb.8 \
 	zfs.8 \
-	zfs-mount-generator.8 \
 	zfs-program.8 \
 	zgenhostid.8 \
 	zinject.8 \
@@ -12,24 +11,18 @@ dist_man_MANS = \
 	zstreamdump.8
 
 nodist_man_MANS = \
-	zed.8
+	zed.8 \
+	zfs-mount-generator.8
 
 EXTRA_DIST = \
-	zed.8.in
+	zed.8.in \
+	zfs-mount-generator.8.in
 
-zed.8: $(srcdir)/zed.8.in
-
-do_subst = $(SED) \
-	-e 's|@libexecdir[@]|$(libexecdir)|g' \
-	-e 's|@runstatedir[@]|$(runstatedir)|g' \
-	-e 's|@sysconfdir[@]|$(sysconfdir)|g'
-
-$(nodist_man_MANS): Makefile
-	$(RM) $@ $@.tmp
-	srcdir=''; \
-	  test -f ./$@.in || srcdir=$(srcdir)/; \
-	  $(do_subst) $${srcdir}$@.in >$@.tmp
-	mv $@.tmp $@
+$(nodist_man_MANS): %: %.in
+	-$(SED) -e 's,@libexecdir\@,$(libexecdir),g' \
+		-e 's,@runstatedir\@,$(runstatedir),g' \
+		-e 's,@sysconfdir\@,$(sysconfdir),g' \
+		$< >'$@'
 
 install-data-local:
 	$(INSTALL) -d -m 0755 "$(DESTDIR)$(mandir)/man8"