Improved readonly conditions

Previously, when a user had imported an existing jail (as created by the Nas4Free wiki), their jail name might have conflicted with an existing jail. Worse, their rc.conf.local file created following the wiki would have placed the jail's rootfolder at a location that depended on the jail_start script's mount_nullfs. Now, if the hostname is a duplicate or if the existing jail's path doesn't exist, then those fields are now read-write.