PATCH: Exclude temp relations from base backup

This is a follow-up patch from the exclude unlogged relations discussion
[1].

The patch excludes temporary relations during a base backup using the
existing looks_like_temp_rel_name() function for identification.

It shares code to identify database directories from [1], so for now
that has been duplicated in this patch to make it independent.  I'll
rebase depending on what gets committed first.

Thanks,
-- 
-David
david@pgmasters.net

[1]
https://www.postgresql.org/message-id/04791bab-cb04-ba43-e9c0-664a4c1ffb2c%40pgmasters.net