Init script fixes

* Fix regression - "OVERLAY_MOUNTS" should have been "DO_OVERLAY_MOUNTS".
* Fix update-rc.d commands in postinst.  Thanx to subzero79@GitHub.
* Fix make sure a filesystem exists before trying to mount in mount_fs()
* Fix local variable usage.
* Fix to read_mtab():
  * Strip control characters (space - \040) from /proc/mounts GLOBALY,
    not just first occurrence.
  * Don't replace unprintable characters ([/-. ]) for use in the variable
    name with underscore. No need, just remove them all together.
* Add check_boolean() to check if a user configure option is
  set ('yes', 'Yes', 'YES' or any combination there of) OR '1'.
  Anything else is considered 'unset'.
* Add a ZFS_POOL_IMPORT to the default config.
  * This is a semi colon separated list of pools to import ONLY.
  * This is intended for systems which have _a lot_ of pools (from
    a SAN for example) and it would be to many to put in the
    ZFS_POOL_EXCEPTIONS variable..
* Add a config option "ZPOOL_IMPORT_OPTS" for adding additional options
  to "zpool import".
* Add documentation and the chance of overriding the ZPOOL_CACHE
  variable in the config file.
* Remove "sort" from find_pools() and setup_snapshot_booting().
  Sometimes not available, and not really necessary.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Issue #3816

etc/init.d/zfs-share.in

@@ -40,11 +40,7 @@ do_depend()
 
 do_start()
 {
-	case "$ZFS_SHARE" in
-		[Oo][Ff][Ff]|[Nn][Oo]|''|0)
-			exit 0
-			;;
-	esac
+	check_boolean "$ZFS_SHARE" || exit 0
 
 	check_module_loaded "zfs" || exit 0
 
@@ -53,11 +49,7 @@ do_start()
 
 do_stop()
 {
-	case "$ZFS_UNSHARE" in
-		[Oo][Ff][Ff]|[Nn][Oo]|''|0)
-			exit 0
-			;;
-	esac
+	check_boolean "$ZFS_UNSHARE" || exit 0
 
 	check_module_loaded "zfs" || exit 0