add "mox localserve" subcommand, for running mox locally for email-related testing/developing

localserve creates a config for listening on localhost for
imap/smtp/submission/http, on port numbers 1000 + the common service port
numbers. all incoming email is accepted (if checks pass), and a few pattern in
localparts are recognized and result in delivery errors.

gendoc.sh

@@ -28,7 +28,9 @@ directory) through the -config flag or MOXCONF environment variable.
 
 EOF
 
-./mox helpall 2>&1
+# setting XDG_CONFIG_HOME ensures "mox localserve" has reasonable default
+# values in its help output.
+XDG_CONFIG_HOME='$userconfigdir' ./mox helpall 2>&1
 
 cat <<EOF
 */