pgsql: Use signals for postmaster death on FreeBSD.

Use signals for postmaster death on FreeBSD.

Use FreeBSD 11.2's new support for detecting parent process death to
make PostmasterIsAlive() very cheap, as was done for Linux in an
earlier commit.

Author: Thomas Munro
Discussion: https://postgr.es/m/7261eb39-0369-f2f4-1bb5-62f3b6083b5e@iki.fi

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/f98b8476cd4a19dfc602ab95642ce08e53877d65

Modified Files
--------------
configure                          | 2 +-
configure.in                       | 2 +-
src/backend/storage/ipc/pmsignal.c | 3 +++
src/include/pg_config.h.in         | 3 +++
src/include/storage/pmsignal.h     | 7 ++++++-
5 files changed, 14 insertions(+), 3 deletions(-)