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