pgsql: Fix handling of HBA ldapserver with multiple hostnames.

Fix handling of HBA ldapserver with multiple hostnames.

Commit 35c0754f failed to handle space-separated lists of alternative
hostnames in ldapserver, when building a URI for ldap_initialize()
(OpenLDAP).  Such lists need to be expanded to space-separated URIs.

Repair.  Back-patch to 11, to fix bug report #15495.

Author: Thomas Munro
Reported-by: Renaud Navarro
Discussion: https://postgr.es/m/15495-2c39fc196c95cd72%40postgresql.org

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/257ef3cd4fec7ca1213f31b660486b492b1c7031

Modified Files
--------------
src/backend/libpq/auth.c    | 42 +++++++++++++++++++++++++++++++++++++-----
src/test/ldap/t/001_auth.pl | 18 +++++++++++++++++-
2 files changed, 54 insertions(+), 6 deletions(-)