timers: Split posix timers restore into two stages

1st stage is -- creating the timers. It may fail if kernel
allocated IDs in a manner we don't expect or runs out of
memory.

2nd stage is -- arm the timers. It cannot fail, since we've
validated the timespecs in advance and should happen after
we've waited for all the other tasks to complete the restore.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>