Forgive trailing semicolons inside of config files

This has been a long-standing annoyance of mine. Who hasn't done something
like this?:

psql> SET random_page_cost = 2.5;
(do some stuff, realize that rpc was too high)

Let's put that inside of postgresql.conf:

#------------------------------------------------------------------------------
# CUSTOMIZED OPTIONS
#------------------------------------------------------------------------------

# Add settings for extensions here

random_page_cost = 2.5;

Boom! Server will not start. Surely, we can be a little more liberal in
what we accept? Attached patch allows a single trailing semicolon to be
silently discarded. As this parsing happens before the logging collector
starts up, the error about the semicolon is often buried somewhere in a
separate logfile or journald - so let's just allow postgres to start up
since there is no ambiguity about what random_page_cost (or any other GUC)
is meant to be set to.

I also considered doing an additional ereport(LOG) when we find one, but
seemed better on reflection to simply ignore it.

Cheers,
Greg