- Make sure random_adaptor accesses happen only when
   random_adaptors_lock is held.
 - Use sx_sleep instead of tsleep in read and write path to allow
   another thread that registers a new random adapter when waiting.
   Assert that random_adaptor is not NULL after reacquiring the lock.
 - Capture EINTR/ERESTART from sx_sleep to allow the blocking cycle be
   stopped when user requests so, while there also make short
   read/write's return 0.
 - Move M_WAITOK allocations out of lock scope.

In collobration with:	kib, markm, ian, jilles
Reviewed by:	kib, markm
Approved by:	so