mount: Move mount_info and ext_mount to mount.h

It's quite unclean while this structure lives
in proc_parse.h, which only have to fill this
structure on procfs read, but real handling
is inside mount.c. Move it as appropriate.

Same time ext_mount structure should be moved
into a header as well with sane @list name
used instead of @l.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>