man: add ip-netns(8) as generation target

Prepare for adding new variable substitutions. Unify the sed rules while
we're at it, since there's no need to write this out 4 times.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

man/man8/.gitignore

@@ -1,4 +1,5 @@
 # these pages are built
 ip-address.8
 ip-link.8
+ip-netns.8
 ip-route.8

man/man8/Makefile

@@ -1,17 +1,11 @@
 # SPDX-License-Identifier: GPL-2.0
-TARGETS = ip-address.8 ip-link.8 ip-route.8
+TARGETS = ip-address.8 ip-link.8 ip-netns.8 ip-route.8
 
 MAN8PAGES = $(TARGETS) $(filter-out $(TARGETS),$(wildcard *.8))
 
 all: $(TARGETS)
 
-ip-address.8: ip-address.8.in
-	sed "s|@SYSCONFDIR@|$(CONFDIR)|g" $< > $@
-
-ip-link.8: ip-link.8.in
-	sed "s|@SYSCONFDIR@|$(CONFDIR)|g" $< > $@
-
-ip-route.8: ip-route.8.in
+%: %.in
 	sed "s|@SYSCONFDIR@|$(CONFDIR)|g" $< > $@
 
 distclean: clean