• 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