pgsql: Revoke support for strxfrm() that write past the specified array

Revoke support for strxfrm() that write past the specified array length.

This formalizes a decision implicit in commit
4ea51cdfe85ceef8afabceb03c446574daa0ac23 and adds clean detection of
affected systems.  Vendor updates are available for each such known bug.
Back-patch to 9.5, where the aforementioned commit first appeared.

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/aaf15ee33a63c582fbb61b67befdd620e85da2ce

Modified Files
--------------
src/backend/main/main.c           |    2 ++
src/backend/utils/adt/pg_locale.c |   58 +++++++++++++++++++++++++++++++++++++
src/backend/utils/adt/selfuncs.c  |   17 +++++------
src/backend/utils/init/postinit.c |    2 ++
src/include/utils/pg_locale.h     |    1 +
5 files changed, 70 insertions(+), 10 deletions(-)