Change zfsunlock for better busybox compatibility

It turns out that there are two versions of Busybox, at least on Ubuntu
18.04.  If you have the busybox-static package installed, you get a
busybox that supports `ps a` and `head`.  If you only have
busybox-initramfs, you don't.  Either way, you have `awk`.

This change should also make this compatible with GNU ps, if you somehow
end up with that in the initramfs environment.

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Andrey Prokopenko <job@terem.fr>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #10307

contrib/initramfs/zfsunlock

@@ -25,7 +25,7 @@ while [ ! -e /run/zfs_unlock_complete ]; do
 		/sbin/zfs load-key "$zfs_fs_name" || true
 	if [ "$(/sbin/zfs get -H -ovalue keystatus "$zfs_fs_name" 2> /dev/null)" = "available" ]; then
 		echo "Password for $zfs_fs_name accepted."
-		zfs_console_askpwd_pid=$(ps a -o pid= -o args | grep -v grep | grep "$zfs_console_askpwd_cmd" | cut -d ' ' -f3 | sort -n | head -n1)
+		zfs_console_askpwd_pid=$(ps | awk '!'"/awk/ && /$zfs_console_askpwd_cmd/ { print \$1; exit }")
 		if [ -n "$zfs_console_askpwd_pid" ]; then
 			kill "$zfs_console_askpwd_pid"
 		fi