734 taskq_dispatch_prealloc() desired

943 zio_interrupt ends up calling taskq_dispatch with TQ_SLEEP
illumos/illumos-gate@5aeb94743e3be0c51e86f73096334611ae3a058e

Essentially FreeBSD taskqueues already operate in a mode that
was added to Illumos with taskq_dispatch_ent change.
We even exposed the superior FreeBSD interface as taskq_dispatch_safe.
Now we just rename taskq_dispatch_safe to taskq_dispatch_ent and
struct struct ostask to taskq_ent_t, so that code differences will be
minimal.

After this change sys/cddl/compat/opensolaris/sys/taskq.h header is no
longer needed.

Note that this commit is not an MFV because the upstream change was not
individually committed to the vendor area.

MFC after: 8 days