Back-patch recent pg_upgrade fixes into 9.2.

Back-patch recent pg_upgrade fixes into 9.2.

This syncs contrib/pg_upgrade in the 9.2 branch with HEAD, except for the
HEAD changes related to converting XLogRecPtr to 64-bit int.  It includes
back-patching these commits:

666d494d19dbd5dc7a177709a2f7069913f8ab89
pg_upgrade: abstract out copying of files from old cluster to new
7afa8bed65ea925208f128048f3a528a64e1319a
pg_upgrade: Run the created scripts in the test suite
ab577e63faf792593ca728625a8ef0b1dfaf7500
Remove analyze_new_cluster.sh on make clean, too
34c02044ed7e7defde5a853b26dcd806c872d974
Fix thinko in comment
088c065ce8e405fafbfa966937184ece9defcf20
pg_upgrade: Fix exec_prog API to be less flaky
f763b77193b04eba03a1f4ce46df34dc0348419e
Fix pg_upgrade to cope with non-default unix_socket_directory scenarios.