Re: Kerberos delegation support in libpq and postgres_fdw

Reviewed v8; largely looking good, though I notice this hunk, which may
arguably be a bug fix, but doesn't appear to be relevant to this specific
patch, so could probably be debated independently (and if a bug, should
probably be backpatched):

diff --git a/contrib/postgres_fdw/option.c b/contrib/postgres_fdw/option.c
index 4229d2048c..11d41979c6 100644
--- a/contrib/postgres_fdw/option.c
+++ b/contrib/postgres_fdw/option.c
@@ -288,6 +288,9 @@ InitPgFdwOptions(void)
  {"sslcert", UserMappingRelationId, true},
  {"sslkey", UserMappingRelationId, true},

+ /* gssencmode is also libpq option, same to above. */
+ {"gssencmode", UserMappingRelationId, true},
+
  {NULL, InvalidOid, false}
  };

That said, should "gssdeleg" be exposed as a user mapping?  (This shows up
in postgresql_fdw; not sure if there are other places that would be
relevant, like in dblink somewhere as well, just a thought.)

Best,

David