Add functions pg_start_backup, pg_stop_backup to create backup label

Add functions pg_start_backup, pg_stop_backup to create backup label
and history files as per recent discussion.  While at it, remove
pg_terminate_backend, since we have decided we do not have time during
this release cycle to address the reliability concerns it creates.
Split the 'Miscellaneous Functions' documentation section into
'System Information Functions' and 'System Administration Functions',
which hopefully will draw the eyes of those looking for such things.