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
------
REL9_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/1a366d51effd0e9f3301d68c2d3dce09c8c43d1f

Modified Files
--------------
configure                           |    3 ++-
configure.in                        |    2 +-
src/backend/postmaster/postmaster.c |   43 +++++++++++++++++++++++++++++++++++
src/include/pg_config.h.in          |    3 +++
src/port/exec.c                     |   12 ++++++++++
5 files changed, 61 insertions(+), 2 deletions(-)