pgsql: Ensure libpq reports a suitable error message on unexpected sock

Ensure libpq reports a suitable error message on unexpected socket EOF.

The EOF-detection logic in pqReadData was a bit confused about who should
set up the error message in case the kernel gives us read-ready-but-no-data
rather than ECONNRESET or some other explicit error condition. Since the
whole point of this situation is that the lower-level functions don't know
there's anything wrong, pqReadData itself must set up the message. But
keep the assumption that if an errno was reported, a message was set up at
lower levels.

Per bug #11712 from Marko Tiikkaja. It's been like this for a very long
time, so back-patch to all supported branches.

Branch
------
REL9_1_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/98144378c4811b71728e5d78c043d6a147e5e0bc

Modified Files
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src/interfaces/libpq/fe-misc.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)