truss: minor cleanup and pedantic warning fixes

Noticed while porting the recent truss compat32 changes to CheriBSD.
This also fixes i386 tracing by zero-extending user addresses instead
of sign-extending them.

Reviewed By: jhb
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D30211

(cherry picked from commit 31dddc6a19042bad303cb3d3aa8f52a2cb22dd77)