libmount: clean up statmount syscall-related functions

libmount: clean up statmount syscall-related functions

* Rename the raw version of the syscall from ul_statmount() to
  ul_statmount_syscall().

* Rename `sys_statmount()` to `ul_statmount()`. The goal is to use the
  same naming convention as `ul_listmount()` for a function that is
  expected to be called from code.

* Move ul_statmount() to be in the same #ifdef block as the rest of
  the statmount code in the header file.

* Add has_statmount() to make it easy to verify the usability of the
  statmount syscall.

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