pgsql: Fix up usage of krb_server_keyfile GUC parameter.

Fix up usage of krb_server_keyfile GUC parameter.

secure_open_gssapi() installed the krb_server_keyfile setting as
KRB5_KTNAME unconditionally, so long as it's not empty.  However,
pg_GSS_recvauth() only installed it if KRB5_KTNAME wasn't set already,
leading to a troubling inconsistency: in theory, clients could see
different sets of server principal names depending on whether they
use GSSAPI encryption.  Always using krb_server_keyfile seems like
the right thing, so make both places do that.  Also fix up
secure_open_gssapi()'s lack of a check for setenv() failure ---
it's unlikely, surely, but security-critical actions are no place
to be sloppy.

Also improve the associated documentation.

This patch does nothing about secure_open_gssapi()'s use of setenv(),
and indeed causes pg_GSS_recvauth() to use it too.  That's nominally
against project portability rules, but since this code is only built
with --with-gssapi, I do not feel a need to do something about this
in the back branches.  A fix will be forthcoming for HEAD though.

Back-patch to v12 where GSSAPI encryption was introduced.  The
dubious behavior in pg_GSS_recvauth() goes back further, but it
didn't have anything to be inconsistent with, so let it be.

Discussion: https://postgr.es/m/2187460.1609263156@sss.pgh.pa.us

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/861e967176e99da9122bb19bc2312c2ecdf6673c

Modified Files
--------------
doc/src/sgml/client-auth.sgml                 |  6 +-----
doc/src/sgml/config.sgml                      | 12 ++++++++---
src/backend/libpq/auth.c                      | 31 +++++++++------------------
src/backend/libpq/be-secure-gssapi.c          | 12 +++++++++--
src/backend/utils/misc/postgresql.conf.sample |  2 +-
5 files changed, 31 insertions(+), 32 deletions(-)