pgsql: Support use of strnlen() in pre-v11 branches.

Support use of strnlen() in pre-v11 branches. Back-patch a minimal subset of commits fffd651e8 and 46912d9b1, to support strnlen() on all platforms without adding any callers. This will be needed by a following bug fix. Branch -- REL_10_STABLE Details --- https://git.postgresql.org/pg/co

pgsql: Support use of strnlen() in pre-v11 branches.

Support use of strnlen() in pre-v11 branches. Back-patch a minimal subset of commits fffd651e8 and 46912d9b1, to support strnlen() on all platforms without adding any callers. This will be needed by a following bug fix. Branch -- REL9_6_STABLE Details --- https://git.postgresql.org/pg/co