libmount: add libmnt_optlist

libmount: add libmnt_optlist

Now libmount internally keeps mount options as strings or flags. This
commit adds a new container to keep parsed mount options in memory.
We need:

 * the same mount options could be mapped to the different mount options
   maps (for example "nosuid" to classic MS_NOSUID or new MOUNT_ATTR_NOSUID)

 * easy to sync flags and strings (e.g. conversion from MS_NOSUID to
   "nosuid" and vice-versa).

 * double linked list is easy to use in while() when browse all options

 * easy to filter options by specific map (MS_*, userspace, etc.)

 * don't hardcode options maps in options parser, support 3th-party
   maps (for example for extensions like dm-verity, etc.)

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