libmodmount:
 * rename {{{check_parent}}} (very bad name) to {{{is_ctlmgr_parent_of_storage_unplug}}}
 * optimize it a bit
 * refs #2572


git-svn-id: file:///var/svn/freetz/trunk@12545 149334a1-2f27-0410-a3b9-fc62619ac1e6