Consistently test for in-use shared memory.

postmaster startup scrutinizes any shared memory segment recorded in
postmaster.pid, exiting if that segment matches the current data
directory and has an attached process.  When the postmaster.pid file was
missing, a starting postmaster used weaker checks.  Change to use the
same checks in both scenarios.  This increases the chance of a startup
failure, in lieu of data corruption, if the DBA does "kill -9 `head -n1
postmaster.pid` && rm postmaster.pid && pg_ctl -w start".  A postmaster
will no longer recycle segments pertaining to other data directories.
That's good for production, but it's bad for integration tests that
crash a postmaster and immediately delete its data directory.  Such a
test now leaks a segment indefinitely.  No "make check-world" test does
that.  win32_shmem.c already avoided all these problems.  In 9.6 and
later, enhance PostgresNode to facilitate testing.  Back-patch to 9.4
(all supported versions).

Reviewed by Daniel Gustafsson and Kyotaro HORIGUCHI.

Discussion: https://postgr.es/m/20130911033341.GD225735@tornado.leadboat.com