Initial onexit handler for OSX, relying on current_proc().

Attempt to change zfs recv to use unix domain socket instead of mkfifo
Use new SPL getf/releasef and roll back to normal ZFS source code
Remove unknown macros
Update zfs recv to use new SPL
Unfix ZOL function name back to ZFS
Use the proper macro for VN_RDWR
Rollback
Remove userland pipe juggle
Remove debug code, change onexit_fd_hold to call getf/releasef from SPL
Merge branch 'onexit'