pgsql: In pg_upgrade, remove dependency on pg_config, as that might not

In pg_upgrade, remove dependency on pg_config, as that might not be in
the non-development install. Instead, use the LOAD mechanism to check
for the pg_upgrade_support shared object, like we do for other shared
object checks.

Backpatch to 9.1.

Report from Àlvaro

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/02b183acb42831bbeba819fde568263f19f2acdc

Modified Files
--------------
contrib/pg_upgrade/check.c | 42 -----------------------------------------
contrib/pg_upgrade/function.c | 15 ++++++++++---
2 files changed, 11 insertions(+), 46 deletions(-)