pgsql: pg_upgrade: warn about extensions that need updating

pg_upgrade:  warn about extensions that need updating

Also create a script that can be run to update them.

Reported-by: Dave Cramer

Discussion: https://postgr.es/m/CADK3HHKawwbOcGwMGnDuAf3-U8YfvTcS8jqDv3UM=niijs3MMA@mail.gmail.com

Backpatch-through: 9.6

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/5e531bb1d368bee9ae6671bdb16312b2d4e64b52

Modified Files
--------------
src/bin/pg_upgrade/check.c      |  2 ++
src/bin/pg_upgrade/pg_upgrade.h |  1 +
src/bin/pg_upgrade/version.c    | 78 +++++++++++++++++++++++++++++++++++++++++
3 files changed, 81 insertions(+)