pgsql: On Darwin, detect and report a multithreaded postmaster.

On Darwin, detect and report a multithreaded postmaster.

Darwin --enable-nls builds use a substitute setlocale() that may start a
thread.  Buildfarm member orangutan experienced BackendList corruption
on account of different postmaster threads executing signal handlers
simultaneously.  Furthermore, a multithreaded postmaster risks undefined
behavior from sigprocmask() and fork().  Emit LOG messages about the
problem and its workaround.  Back-patch to 9.0 (all supported versions).

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/83fb1ca5cf393f3a12930bd9275a711cd858c823

Modified Files
--------------
configure                           |    2 +-
configure.in                        |    2 +-
src/backend/postmaster/postmaster.c |   43 +++++++++++++++++++++++++++++++++++
src/common/exec.c                   |   12 ++++++++++
src/include/pg_config.h.in          |    3 +++
5 files changed, 60 insertions(+), 2 deletions(-)