libmount: (mount) improve syscalls status handling

libmount: (mount) improve syscalls status handling

The new kernel API is composed from more syscalls, but libmount
originally assumes one syscall and one place where we need to remember
return value of the syscall. It seems the best will be to set the
status after each mount related syscall call (fsopen(), move_mount(),
...) until we reach error or end of mount  process.

For better error messages the last failed mount related syscall name
will be recorded in cxt->syscall_name.

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