Per-column collation, work in progress

Following up on my previous patch [0], here is a fairly complete
implementation of this feature. The general description and
implementation outline of the previous message still apply. This patch
contains documentation and regression tests, which can serve as further
explanations.

As this patch touches pretty much everything in the system, there are
probably countless bugs and bogosities, some of which I have marked with
FIXME, TODO, etc. But all the functionality is basically there, so it's
time someone else gives this a serious examination.

Note: As previously, regression tests only work with "make check
MULTIBYTE=UTF8" and the feature overall only works on Linux/glibc.

[0]
http://archives.postgresql.org/message-id/1279045531.32647.14.camel@vanquo.pezone.net