MDL-60923 backup: Added $CFG->backuptempdir

The new setting will allow to host the temporary backup files
into a specific target directory. Defaults to '$CFG->tempdir/backup'.

Calling make_backup_temp_directory() checks that the required sub-directory
will be properly created under the new target directory.

admin/cli/install.php

@@ -442,6 +442,7 @@ if ($interactive) {
     }
 }
 $CFG->tempdir       = $CFG->dataroot.'/temp';
+$CFG->backuptempdir = $CFG->tempdir.'/backup';
 $CFG->cachedir      = $CFG->dataroot.'/cache';
 $CFG->localcachedir = $CFG->dataroot.'/localcache';