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
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master

Details
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http://git.postgresql.org/pg/commitdiff/214c7a4f0b1784ce855512c2961b09c9f51dafd8

Modified Files
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contrib/worker_spi/worker_spi.c | 7 ++++++-
src/backend/replication/walreceiver.c | 4 ++++
src/backend/replication/walsender.c | 18 ++++++++++++------
3 files changed, 22 insertions(+), 7 deletions(-)