split rm_name and rm_desc out of rmgr.c

Hi,

pg_xlogdump needs access to the *_desc functions for each rmgr.  We
already moved forward quite a bit by splitting those functions out of
their containing files; so now they are compilable separately.  Good.
The remaining task is enabling the code to find those functions in the
first place; currently, the function pointers live in rmgr.c which is
not compilable by frontend code because it contains pointers to other
functions.  Hence the attached patch splits RmgrData into two; the names
and rm_desc functions go into a new file which can be compiled easily by
frontend.

Proposed patch attached.

This comes from 
http://www.postgresql.org/message-id/20130204180327.GH4963@alvh.no-ip.org
which is part of the pg_xlogdump patch in commitfest.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services