Add support for displaying thread IDs to truss(1).

Add support for displaying thread IDs to truss(1).

- Consolidate duplicate code for printing the metadata at the start of
  each line into a shared function.
- Add an -H option which will log the thread ID of the relevant thread
  for each event.

While here, remove some extraneous calls to clock_gettime() in
print_syscall() and print_syscall_ret().  The caller of print_syscall_ret()
always updates the current thread's "after" time before it is called.

Reviewed by:	kib
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D5363