Problem Statement
ZFS Channel program scripts currently require a timeout, so that hung
or long-running scripts return a timeout error instead of causing ZFS
to get wedged.  This limit can currently be set up to 100 million Lua
instructions. Even with a limit in place, it would be desirable to
have a sys admin (support engineer) be able to cancel a script that is
taking a long time.

Proposed Solution
Make it possible to abort a channel program by sending an interrupt
signal.In the underlying txg_wait_sync function, switch the cv_wait to
a cv_wait_sig to catch the signal. Once a signal is encountered, the
dsl_sync_task function can install a Lua hook that will get called
before the Lua interpreter executes a new line of code. The
dsl_sync_task can resume with a standard txg_wait_sync call and wait
for the txg to complete. Meanwhile, the hook will abort the script and
indicate that the channel program was canceled. The kernel returns a
EINTR to indicate that the channel program run was canceled.