pgsql: Add a function to get the authenticated user ID.

Add a function to get the authenticated user ID.

Previously, this was not exposed outside of miscinit.c. It is needed
for the pending pg_background patch, and will also be needed for
parallelism. Without it, there's no way for a background worker to
re-create the exact authentication environment that was present in the
process that started it, which could lead to security exposures.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/5ac372fc1a7cc673cc7d4cf26ba651d52495b27a

Modified Files
--------------
src/backend/utils/init/miscinit.c | 10 ++++++++++
src/include/miscadmin.h | 1 +
2 files changed, 11 insertions(+)