The previous commit backported the PostgreSQL 9.3 pg_upgrade code
base and overwrote the hacks to allow for upgrading a Greenplum
cluster. This commit brings back the patchsets that were lost, and
adapts them to the new pg_upgrade code as well as mildly refactors
them to take advantage of new opportunities.

This attempts to minimize the diff to upstream in preparation of
merging with a later version of pg_upgrade. In some cases this
re-introduces quirks from small style cleanups we've performed,
but it will only be for a short period until the merge happens.

PostgreSQL have in recent versions moved to just having a version.c
file for version specific function. In this commit we move Greenplum
specific code to version_gp.c, since we would do that sooner or later
anyways.

No functionality is altered with this, this is limited to refactoring.