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.
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