ssh_filter_btrbk.sh: use more common exit statuses

POSIX designates a number of exit statuses around `127` for special use and GNU
a few further.
Further, using values >127 is rather uncommon for normal use-cases.

Use `1` when the SSH command was rejected and `2` when the program’s arguments
could not be parsed).

Since this might at least in principle be used by 3rd-party programs, added a
specific note to the changelog.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>

ChangeLog

@@ -1,3 +1,9 @@
+btrbk-current
+
+  * ssh_filter_btrbk.sh uses new exit statuses on failures (1 when the
+    SSH command was rejected, 2 when the program’s arguments could not
+    be parsed).
+
 btrbk-0.32.5
 
   * Correct handling of zero-size raw info file (close #491).

ssh_filter_btrbk.sh

@@ -42,7 +42,7 @@ reject_and_die()
     local reason="$1"
     log_cmd 'auth.err' 'btrbk REJECT' "$reason"
     printf 'ERROR: ssh_filter_btrbk.sh: ssh command rejected: %s: %s\n' "$reason" "$SSH_ORIGINAL_COMMAND" 1>&2
-    exit 255
+    exit 1
 }
 
 run_cmd()
@@ -166,7 +166,7 @@ while [ "$#" -ge 1 ]; do
 
       *)
           printf 'ERROR: ssh_filter_btrbk.sh: failed to parse command line option: %s\n' "$key" 1>&2
-          exit 255
+          exit 2
           ;;
     esac
     shift