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
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master

Details
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http://git.postgresql.org/pg/commitdiff/be8b06c364db315258fb074e9bfbf5db04ab2c5f

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(-)