[debian] grep_dd_sigs: add usage message

debian/grep_dd_sigs

@@ -15,6 +15,15 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+if [ $# -ne 1 ] ; then
+	echo "Usage:"
+	echo "	$0 <gpg_key>"
+	echo "<gpg_key> can be either string that identifies a GPG key: key"
+	echo "ID, fingerprint, UID, etc. (see 'HOW TO SPECIFY A USER ID' in"
+	echo "the gpg's manpage)."
+	exit 1
+fi
+
 sigs=$(gpg --with-colons --check-sigs $1 \
 	| grep "sig:\!" \
 	| sed -r "s/sig:\!::[[:digit:]]+:([[:alnum:]]+):.*$/\1/")