If the user provides a specific PID to trace, it doesn't really make
sense to keep running if the tracee terminates. This patch makes
bpftrace exit cleanly if the tracee terminates.

I spent quite a bit of time thinking about the generic problem of
figuring out when an arbitrary pid terminates. After some experiments,
here is what I've learned:
* wait(2) and waitpid(2) can only wait on child processes (duh)
* epoll(2) does not support procfs (or other pseudo filesystems)
* inotify does not support procfs either b/c procfs changes are not made
through the filesystem (by another userspace entity)
* ptrace with PTRACE_SEIZE might work but might have extra overhead on
the tracee
* the netlink interface for process state changes seems a bit overkill
* the only sane solution (AFAICT) is to poll /proc/PID/ for changes

Thus, I've made some minor changes to the main event loop to support
polling procfs.

Test Plan:
Make sure non-pid-specific tracing still works:
```
$ sudo ./build/src/bpftrace -e 'uretprobe:/bin/bash:readline { printf("read a line\n"); }'
[sudo] password for dlxu:
Attaching 1 probe...
read a line
read a line
read a line
read a line
read a line
read a line
read a line
read a line
^C

```

Verify pid-specific tracing (ie usdt) exits on tracee termination:
```
// in window 1
$ ./python -q

// in window 2
$ sudo ~/dev/bpftrace/build/src/bpftrace -p $(pidof python) -e 'usdt:/home/dlxu/dev/cpython/python:function__entry { printf("%s %s\n", str(arg0), str(arg1)) }'
[sudo] password for dlxu:
Attaching 1 probe...
<stdin> <module>

// in window 1
>>> print('wow')
wow
>>>

// verify bpftrace has exited in window 2

```