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_1_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/03f06ff383eaeb80e1ad1f2aa496ab0cb283f3da

Modified Files
--------------
src/backend/replication/walsender.c |   18 ++++++++++++------
1 file changed, 12 insertions(+), 6 deletions(-)