pgsql: Ensure that we retry rather than erroring out when send() or

Log Message:
-----------
Ensure that we retry rather than erroring out when send() or recv() return
EINTR; the stats code was failing to do this and so were a couple of places
in the postmaster.  The stats code assumed that recv() could not return EINTR
if a preceding select() showed the socket to be read-ready, but this is
demonstrably false with our Windows implementation of recv(), and it may
not be the case on all Unix variants either.  I think this explains the
intermittent stats regression test failures we've been seeing, as well
as reports of stats collector instability under high load on Windows.

Backpatch as far as 8.0.

Modified Files:
--------------
    pgsql/src/backend/postmaster:
        pgstat.c (r1.135 -> r1.136)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.135&r2=1.136)
        postmaster.c (r1.494 -> r1.495)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.494&r2=1.495)