pgsql: Move routine definitions of xlogarchive.c to a new header file

Move routine definitions of xlogarchive.c to a new header file

The definitions of the routines defined in xlogarchive.c have been part
of xlog_internal.h which is included by several frontend tools, but all
those routines are only called by the backend. More cleanup could be
done within xlog_internal.h, but that's already a nice cut.

This will help a follow-up patch for pg_rewind where handling of
restore_command is added for frontends.

Author: Alexey Kondratov, Michael Paquier
Reviewed-by: Álvaro Herrera, Alexander Korotkov
Discussion: /messages/by-id/a3acff50-5a0d-9a2c-b3b2-ee36168955c1@postgrespro.ru

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/616ae3d2b0566e91b49f301bf08410a9972fed93

Modified Files
--------------
src/backend/access/transam/timeline.c | 1 +
src/backend/access/transam/xlog.c | 1 +
src/backend/access/transam/xlogarchive.c | 1 +
src/backend/replication/walreceiver.c | 1 +
src/include/access/xlog_internal.h | 18 ----------------
src/include/access/xlogarchive.h | 35 ++++++++++++++++++++++++++++++++
6 files changed, 39 insertions(+), 18 deletions(-)