libmount: (mount) use independent hooks

libmount: (mount) use independent hooks

It seems better to reuse functions (hooks) to implement various
ways the mount is done. For example, attach to the tree is used
for move, bind, and new mount; set VFS flags is usable for all
operations, etc.

Complete mount(8) is a set (chain) of hooks that is possible to mix to
get the wanted result.

The new API allow for example things like

	mount --move /A /B -oro,noexec

because it's

	- open_tree()
	- mount_setattr()
	- move_tree()

Note that the hooks allow extend it to new functionality  without core
code modification (for example, X-mount.idmap= append a new hook to
call another mount_setattr()).

Signed-off-by: Karel Zak <kzak@redhat.com>