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

doc/src/sgml/client-auth.sgml

@@ -1265,11 +1265,7 @@ omicron         bryanh                  guest1
 
    <para>
     The location of the server's keytab file is specified by the <xref
-    linkend="guc-krb-server-keyfile"/> configuration
-    parameter. The default is
-    <filename>FILE:/usr/local/pgsql/etc/krb5.keytab</filename>
-    (where the directory part is whatever was specified
-    as <varname>sysconfdir</varname> at build time).
+    linkend="guc-krb-server-keyfile"/> configuration parameter.
     For security reasons, it is recommended to use a separate keytab
     just for the <productname>PostgreSQL</productname> server rather
     than allowing the server to read the system keytab file.