OpenFile() Permissions Refactor

Hackers,

While working on the patch to allow group reads of $PGDATA I refactored
the various OpenFile() functions to use default/global permissions
rather than permissions defined in each call.

I think the patch stands on its own regardless of whether we accept the
patch to allow group permissions (which won't make this CF). We had a
couple different ways of defining permissions (e.g. 0600 vs S_IRUSR |
S_IWUSR) and in any case it represented quite a bit of duplication.
This way seems simpler and less error-prone.

I have intentionally not touched the open/fopen/mkdir calls in these
files since they are specialized and require per-instance consideration
(if they are changed at all):

backend/postmaster/fork_process.c
backend/postmaster/postmaster.c
backend/utils/error/elog.c
backend/utils/init/miscinit.c

All tests pass but it's possible that I've missed something or changed
something that shouldn't be changed.

I'll submit the patch to the 2017-09 CF.

Thanks,
--
-David
david(at)pgmasters(dot)net