pgsql: Fix configure probe for sys/ucred.h.

Fix configure probe for sys/ucred.h.

The configure script's test for  did not work on OpenBSD,
because on that platform  has to be included first.
As a result, socket peer authentication was disabled on that platform.
Problem introduced in commit be4585b1c27ac5dbdd0d61740d18f7ad9a00e268.

Andres Freund, slightly simplified by me.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/3ba763bd95b6eb027bc87e73806241b460807091

Modified Files
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configure    |   76 ++++++++++++++++++++++++++++++++++++++++++++++++++++++----
configure.in |   14 ++++++++---
2 files changed, 83 insertions(+), 7 deletions(-)