Split out list of XLog resource managers

The new rmgrlist.h header, containing all necessary data
about built-in resource managers, allows other pieces of code to
access them.

In particular, this allows a future pg_xlogdump program to extract
rm_desc function pointers, without having to keep a duplicate list of
them.
Enable building with Microsoft Visual Studio 2012.

Backpatch to release 9.2

Brar Piening and Noah Misch, reviewed by Craig Ringer.