truss: Fix display of shm_open(SHM_ANON, ...).

Currently truss(1) shows shm_open(SHM_ANON, ...) as shm_open("(null)", ...).
Detect the special value and display it by name.

Reviewed by: jhb, allanjude, tuexen
Approved by: mjg (mentor)
MFC with: r339224
Differential Revision: https://reviews.freebsd.org/D17461