Rearrange extension-related views as per recent discussion.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 555353c0c59ada35ae59c8a76186e98d123fa8b3
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2011-02-15T00:22:36Z
Releases: 9.1.0
Rearrange extension-related views as per recent discussion.

The original design of pg_available_extensions did not consider the
possibility of version-specific control files.  Split it into two views:
pg_available_extensions shows information that is generic about an
extension, while pg_available_extension_versions shows all available
versions together with information that could be version-dependent.
Also, add an SRF pg_extension_update_paths() to assist in checking that
a collection of update scripts provide sane update path sequences.

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