procd: procd_send_signal use signal names

Usage documentation for 'procd_send_signal' states "The signal is SIGHUP
by default, and must be specified by NAME."  Make actual behaviour match
the stated documented behaviour.

https://wiki.openwrt.org/inbox/procd-init-scripts

Suggested-by: Jo-Philip Wich <jow@mein.io>
Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

package/system/procd/files/procd.sh

@@ -413,6 +413,10 @@ _procd_send_signal() {
 	local instance="$2"
 	local signal="$3"
 
+	case "$signal" in
+		[A-Z]*)	signal="$(kill -l "$signal" 2>/dev/null)" || return 1;;
+	esac
+
 	json_init
 	json_add_string name "$service"
 	[ -n "$instance" -a "$instance" != "*" ] && json_add_string instance "$instance"