Update 035_valid_backup_methods.sh

Simplified prep/default/035_valid_backup_methods.sh
to make it work more predictable and fail-safe, cf.
#2353 (comment)

usr/share/rear/prep/default/035_valid_backup_methods.sh

@@ -1,28 +1,16 @@
 
-# For "rear mkbackup" and also for "rear mkrescue" (also the latter
-# because for most external backup methods only "rear mkrescue" is used)
-# check what BACKUP methods are mentioned in the form BACKUP=method in default.conf
-# and error out when a BACKUP method is not found this way in default.conf
+# For "rear mkbackup" and also for "rear mkrescue"
+# ( also the latter because for most external backup methods only "rear mkrescue"
+#   is used because most external backup methods do not implement making a backup
+#   but only implement to restore a backup see 'BACKUP SOFTWARE INTEGRATION' in "man rear" )
+# check that the BACKUP method implements a matching backup restore method
+# i.e. check that a usr/share/rear/restore/$BACKUP directory exists
+# and error out when a BACKUP method seems to not support a backup restore
 # to ensure that the user cannot specify a non-working BACKUP in /etc/rear/local.conf
-# and to ensure that each implemented BACKUP method is mentioned in default.conf
-# to have a minimum documentation about what BACKUP methods are implemented in ReaR
 # see https://github.com/rear/rear/issues/914
 # and https://github.com/rear/rear/issues/159
 # and https://github.com/rear/rear/issues/2337#issuecomment-596471615
 
-local backup_method
-local valid_backup_methods=()
-
-# That 'grep|cut|awk' pipe may find more words than actually valid backup methods
-# e.g. assume default.conf contains "using BACKUP=QQQ does not work"
-# and it may not find an actually valid backup method
-# e.g. assume a valid backup method FOO is only mentioned as "use BACKUP=FOO with BACKUP_URL"
-# so a more fail-safe method to autodetect actually valid backup methods may be needed in the future:
-for backup_method in $( grep 'BACKUP=' $SHARE_DIR/conf/default.conf | grep -v '_' | cut -d= -f2 | awk '{print $1}' | sort -u ) ; do
-    valid_backup_methods+=( "$backup_method" )
-done
-
-if ! grep -q "$BACKUP" <<< $( echo ${valid_backup_methods[@]} ) ; then
-    Error "The BACKUP method '$BACKUP' is not known to ReaR."
+if ! test -d "$SHARE_DIR/restore/$BACKUP" ; then
+    Error "The BACKUP method '$BACKUP' is not supported (no $SHARE_DIR/restore/$BACKUP directory)"
 fi
-