udev_rules_parse_file: issue diagnostics about duplicate LABEL tokens

When a rules contains several LABEL tokens, the parser used to silently
discard all of them besides the last one without any diagnostics at all.
It's time to break the vow of silence and let the parser issue a warning.

test/units/testsuite-17.11.sh

@@ -262,6 +262,19 @@ ${rules}: udev rules check failed
 EOF
 assert_1 "${rules}"
 
+cat >"${rules}" <<'EOF'
+GOTO="a"
+LABEL="a", LABEL="b"
+EOF
+cat >"${exp}" <<EOF
+${rules}:2 Contains multiple LABEL keys, ignoring LABEL="a".
+${rules}:1 GOTO="a" has no matching label, ignoring
+${rules}:1 The line takes no effect any more, dropping
+${rules}:2 LABEL="b" is unused.
+${rules}: udev rules check failed
+EOF
+assert_1 "${rules}"
+
 # udevadm verify --root
 sed "s|sample-[0-9]*.rules|${workdir}/${rules_dir}/&|" sample-*.exp >"${workdir}/${exp}"
 cd -