pbio: Add locking and remove D_NEEDGIANT.

Use a sx lock to permit uiomove directly into/out of the the per-port
buffers. In addition, the sx lock provides a stronger guarantee that
I think this driver wants which is to single-thread read and write
calls even while paused. Finally, replace tsleep with dummy wait
channels with calls to pause_sig to more clearly communicate the
intent.

Differential Revision: https://reviews.freebsd.org/D35081