Improve error message wording

The wording changes applied in 0ac5ad513 were universally disliked.

Per gripe from Andrew Dunstan
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.