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: https://postgr.es/m/a3acff50-5a0d-9a2c-b3b2-ee36168955c1@postgrespro.ru