zfsbootmenu: add zbm.waitfor argument handler

With keyfiles being stored on USB drives, the boot process can be
non-deterministic due to intermittently missing /dev entries. Users can
now provide one or more devices that must be present before the pool
import process can start. Unlike dracut, you can simply hit the escape
key to cancel this check.

zfsbootmenu/libexec/zfsbootmenu-init

@@ -90,19 +90,19 @@ if [ -n "${zbm_waitfor_devices}" ]; then
     for device in "${user_devices[@]}"; do
       case "${device}" in
         /dev/*)
-          ((EXPECTED++))
+          ((EXPECTED=EXPECTED+1))
           if [ -e "${device}" ] ; then
-            ((FOUND++))
+           ((FOUND=FOUND+1))
           else
             missing+=( "$device" )
           fi
           ;;
         *=*)
-          ((EXPECTED++))
+          ((EXPECTED=EXPECTED+1))
           path_prefix="/dev/disk/by-${device%=*}"
           checkfor="${path_prefix,,}/${device##*=}"
           if [ -e "${checkfor}" ] ; then
-            ((FOUND++))
+            ((FOUND=FOUND+1))
           else
             missing+=( "$device" )
           fi