PT_CONTINUE and PT_STEP are process-scope on FreeBSD

Although ptrace() can be passed a PID or TID for PT_CONTINUE and PT_STEP,
the kernel operates on all threads in the process in both cases.  (See
the FOREACH_THREAD_IN_PROC in FreeBSD's sys_process.c:kern_ptrace.)

Make this clear by using the PID from the ProcessMonitor instance.

llvm-svn: 195656