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
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/28155746150f584e8147e2e3f68fec8c0d819cbf

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