libmount: add support for statmount()

libmount: add support for statmount()

* introduce libmnt_statmnt object to store statmount mask and buffer
  (etc.)

* add API for libmnt_fs to retrieve data from the kernel using
  statmount()

* support on-demand statmount() from old mnt_fs_get_...() functions

* allow libmnt_statmnt to be shared and reused between libmnt_fs
  instances

* allow libmnt_statmnt to be assigned and used for filesystems in the
  table (assign libmnt_statmnt to the table to make it usable for all
  filesystems).

* allow temporary disabling of fetching data from the kernel to avoid
  unwanted recursion in certain use-cases

* support namespaces for statmount() based on the libmnt_fs namespace
  ID setting

* allow user-defined statmount masks to overwrite the default

* add a sample program

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