Support run-time configuration of the PIPE_MINDIRECT threshold.
PIPE_MINDIRECT determines at what (blocking) write size one-copy
optimizations are applied in pipe(2) I/O. That threshold hasn't
been tuned since the 1990s when this code was originally
committed, and allowing run-time reconfiguration will make it
easier to assess whether contemporary microarchitectures would
prefer a different threshold.

(On our local RPi4 baords, the 8k default would ideally be at least
32k, but it's not clear how generalizable that observation is.)

MFC after: 3 weeks
Reviewers: jrtc27, arichardson
Differential Revision: https://reviews.freebsd.org/D29819