sysutils/py-prometheus-zfs: Initialize rcvar

The rcvar (zfsprom_enable) must be initialized. Otherwise, rc(8)
complains with the following message:

    /etc/rc: WARNING: $zfsprom_enable is not set properly - see rc.conf(5).

sysutils/py-prometheus-zfs/Makefile

@@ -3,7 +3,7 @@
 PORTNAME=	prometheus-zfs
 DISTVERSIONPREFIX=	v
 DISTVERSION=	1.0.3
-PORTREVISION=	1
+PORTREVISION=	2
 CATEGORIES=	sysutils python
 PKGNAMEPREFIX=	${PYTHON_PKGNAMEPREFIX}
 

sysutils/py-prometheus-zfs/files/zfsprom.in

@@ -21,6 +21,8 @@ rcvar="zfsprom_enable"
 
 load_rc_config "${name}"
 
+: "${zfsprom_enable:=NO}"
+
 pidfile="/var/run/${name}.pid"
 procname="%%PREFIX%%/sbin/zfsprom.py"
 command_interpreter="%%PYTHON_CMD%%"