criu: uffd runtime detection

Now that userfaultfd/lazy-pages support is enable all the time, this
adds runtime detection of userfaultfd. On a system without the
userfaultfd syscall following is printed:

uffd daemon:

(00.000004) Error (uffd.c:176): lazy-pages: Runtime detection of userfaultfd failed on this system.
(00.000024) Error (uffd.c:177): lazy-pages: Processes cannot be lazy-restored on this system.

or criu restore

(00.457047)   6858: Error (uffd.c:176): lazy-pages: Runtime detection of userfaultfd failed on this system.
(00.457049)   6858: Error (uffd.c:177): lazy-pages: Processes cannot be lazy-restored on this system.

Signed-off-by: Adrian Reber <areber@redhat.com>
Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>