improve documentation

The README was from the time of original software design, and the way the
f5gs is now used has somewhat changed.  The same is reflected in manual
page.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
make manual page time stamp to be same as build date

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
use more descriptive option names

The --listen was good option to bind server process to an interface, but
not descriptive in context of requesting state information from remote
server, therefore --address is better.

Secondly --state was too vague.  One might think it could take new state
as argument, while it only affects to directory where state file is
stored.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
use word only word 'status' where 'state' and 'state' co-existed

And few other wording improvements to --help output.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>