pgsql: Allow "-C variable" and "--describe-config" even to root users.

Allow "-C variable" and "--describe-config" even to root users.

There's no really compelling reason to refuse to do these read-only,
non-server-starting options as root, and there's at least one good
reason to allow -C: pg_ctl uses -C to find out the true data directory
location when pointed at a config-only directory.  On Windows, this is
done before dropping administrator privileges, which means that pg_ctl
fails for administrators if and only if a config-only layout is used.

Since the root-privilege check is done so early in startup, it's a bit
awkward to check for these switches.  Make the somewhat arbitrary
decision that we'll only skip the root check if -C is the first switch.
This is not just to make the code a bit simpler: it also guarantees that
we can't misinterpret a --boot mode switch.  (While AuxiliaryProcessMain
doesn't currently recognize any such switch, it might have one in the
future.)  This is no particular problem for pg_ctl, and since the whole
behavior is undocumented anyhow, it's not a documentation issue either.
(--describe-config only works as the first switch anyway, so this is
no restriction for that case either.)

Back-patch to 9.2 where pg_ctl first began to use -C.

MauMau, heavily edited by me

Branch
------
REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/6d25eb314a631871c1df1de563253a78ce706250

Modified Files
--------------
src/backend/main/main.c |   26 +++++++++++++++++++++++---
src/bin/pg_ctl/pg_ctl.c |    8 +++++---
2 files changed, 28 insertions(+), 6 deletions(-)