manpages: Put version number instead of date at bottom of page.

manpages: Put version number instead of date at bottom of page.

We're really good about keeping manpages up to date, but terrible at
updating the dates at the bottom of the manpages.  So, instead of using
manually updated dates, this commit switches to using automatically updated
version numbers.

We can only use automatically updated version numbers for manpages that
we preprocess, that is, the manpages whose sources end with ".in".  This
excludes a couple of manpages that don't actually get installed with OVS,
such as the manpages for ovs-ctl and ovsdb-idlc.  This commit doesn't
change those manpages.  It does change the ovs-bugtool manpage to one that
is preprocessed so that we can use the version there.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>