[PATCH] Find additional connection service files in pg_service.conf.d directory

I love centralizing connection service definitions in
<sysconfdir>/pg_service.conf, but for a
large enterprise, sometimes we have multiple sets of connection
service definitions
independently managed.

The convention widely adopted for this type of thing is to allow
multiple config files to be in
a directory, usually the '.d' version of the config filename. See, for example:

http://blog.siphos.be/2013/05/the-linux-d-approach/
https://lists.debian.org/debian-devel/2010/04/msg00352.html

This patch adds an extra search for service connection definitions if
the current places fail
to find the service (~/.pg_service.conf <sysconfdir>/pg_service.conf).
It will then search
every readable file in the directory <sysconfdir>/pg_service.conf.d.

What do you think?

Curt