config: flip return value of store_write_*()

config: flip return value of store_write_*()

The store_write_section() and store_write_pairs() functions
are basically high-level wrappers around write(). But their
return values are flipped from our usual convention, using
"1" for success and "0" for failure.

Let's flip them to follow the usual write() conventions and
update all callers. As these are local to config.c, it's
unlikely that we'd have new callers in any topics in flight
(which would be silently broken by our change). But just to
be on the safe side, let's rename them to just
write_section() and write_pairs().  That also accentuates
their relationship with write().

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>