- 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
Do not try to create a /dev/log symlink in a jail.

PR:	179828
Fix up some session-related races in devfs.

One was introduced with r272596, the rest was there to begin with.

Noted by: jhb
filedesc: create a dedicated zone for struct filedesc0

Currently sizeof(struct filedesc0) is 1096 bytes, which means allocations from
malloc use 2048 bytes.

There is no easy way to shrink the structure <= 1024 an it is likely to grow in
the future.
Don't assert random_adaptors_lock in random_adaptor_read_rate().

Reported by:	many
Pointy hat to:	delphij (for not testing the commit with WITNESS)
Approved by:	so