Fix FTP share issue. Remove useless mntentref config fields.

git-svn-id: svn://svn.code.sf.net/p/openmediavault/code@1480 3c3d7b80-5828-415e-9f61-280b98178fff
Allow plugin developers to create hidden mntent database entries. Those entries won't be written to /etc/fstab.

git-svn-id: svn://svn.code.sf.net/p/openmediavault/code@1481 3c3d7b80-5828-415e-9f61-280b98178fff