fail(9): Upstreaming some fail point enhancements

This is several year's worth of fail point upgrades done at EMC Isilon. They
are interdependent enough that it makes sense to put a single diff up for them.
Primarily, we added:

• Changing all mainline execution paths to be lockless, which lets us use fail points in more sleep-sensitive areas, and allows more parallel execution
• A number of additional commands, including 'pause' that lets us do some interesting deterministic repros of race conditions
• The ability to dump the stacks of all threads sleeping on a fail point
• A number of other API changes to allow marking up the fail point's context in the code, and firing callbacks before and after execution
• A man page update

Submitted by: Matthew Bryan <matthew.bryan@isilon.com>
Reviewed by: cem (earlier version), jhb, kib, pho
With feedback from: bdrewery
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D5427