Improve manpage checking rule.

Improve manpage checking rule.

The coverage of the previous version of this rule was incomplete because
$(MANS) does not include $(noinst_man_MANS).   (Also, $(MANS) is
undocumented.)  Writing it out as the list of manpages variables that
Open vSwitch uses is better.

Also, the previous version of the rule didn't actually signal an error
when warnings appeared.  It looks like troff doesn't have a way to indicate
warnings as part of its exit status, so instead we grep the output for
"warning".