pgsql: Fix some more bugs in signal handlers and process shutdown logic

Fix some more bugs in signal handlers and process shutdown logic.

WalSndKill was doing things exactly backwards: it should first clear
MyWalSnd (to stop signal handlers from touching MyWalSnd->latch),
then disown the latch, and only then mark the WalSnd struct unused by
clearing its pid field.

Also, WalRcvSigUsr1Handler and worker_spi_sighup failed to preserve
errno, which is surely a requirement for any signal handler.

Per discussion of recent buildfarm failures. Back-patch as far
as the relevant code exists.

Branch
------
REL9_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/6f1a4077310e97fe32379263918a31b95f85e2ed

Modified Files
--------------
contrib/worker_spi/worker_spi.c | 6 +++++-
src/backend/replication/walreceiver.c | 4 ++++
src/backend/replication/walsender.c | 18 ++++++++++++------
3 files changed, 21 insertions(+), 7 deletions(-)