Add support for alpine linux

Both Alpine Linux and Gentoo use OpenRC so we share its logic

Signed-off-by: Carlo Landmeter <clandmeter@gmail.com>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4386

etc/init.d/Makefile.am

@@ -21,7 +21,7 @@ $(init_SCRIPTS) $(initconf_SCRIPTS) $(initcommon_SCRIPTS): $(EXTRA_DIST)
 	  else \
 		NFS_SRV=nfs; \
 	  fi; \
-	  if [ -e /etc/gentoo-release ]; then \
+	  if [ -e /sbin/openrc-run ]; then \
 		SHELL=/sbin/runscript; \
 	  else \
 		SHELL=/bin/sh; \

etc/init.d/zfs-import.in

@@ -308,7 +308,7 @@ do_start()
 
 # ----------------------------------------------------
 
-if [ ! -e /etc/gentoo-release ]
+if [ ! -e /sbin/openrc-run ]
 then
 	case "$1" in
 		start)

etc/init.d/zfs-mount.in

@@ -199,7 +199,7 @@ do_stop()
 
 # ----------------------------------------------------
 
-if [ ! -e /etc/gentoo-release ]
+if [ ! -e /sbin/openrc-run ]
 then
 	case "$1" in
 		start)

etc/init.d/zfs-share.in

@@ -58,7 +58,7 @@ do_stop()
 
 # ----------------------------------------------------
 
-if [ ! -e /etc/gentoo-release ]; then
+if [ ! -e /sbin/openrc-run ]; then
 	case "$1" in
 		start)
 			do_start

etc/init.d/zfs-zed.in

@@ -98,7 +98,7 @@ do_reload()
 
 # ----------------------------------------------------
 
-if [ ! -e /etc/gentoo-release ]; then
+if [ ! -e /sbin/openrc-run ]; then
 	case "$1" in
 		start)
 			do_start