jhb pointed out that tanimura is working on patches to protect p_fd,
which is currently not protected yet used outside of Giant due to
the struct filedesc pushdown. While a sub-set of the overall proc locking
task, this appears to be seperable, and in progress, so break it out
into its own task with responsible of 'tanimura'. Currently, there
can be panics under high load with a corrupted p_fd as a result of this
lack of locking.

Bugs reported by: fenner