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
57cd52ca4c