This patch adds a command running option to bpftrace. The user can now
run something like:

    ./bpftrace -e '...' -c 'sleep 5'

which is a convenience wrapper around something like:

    sleep 5 & ./bpfrace -e '...' -p `pidof sleep`

`-c` is better because it:
* ensures a tighter tracing range around CMD (ie we trace less of the
system while it is not running CMD)
* makes bpftrace exit (which is convenient) when CMD terminates
    * previously, it was not possible to get a full trace of CMDs
    execution and have bpftrace exit upon CMD termination

Test Plan:
Trivial successful example:
```
$ sudo ./build/src/bpftrace -e 'tracepoint:syscalls:sys_enter_nanosleep
{ printf("%s nanoslept\n", comm); }' -c '/bin/sleep 1'
[sudo] password for dlxu:
chdir(/lib/modules/4.19.8-200.fc28.x86_64/build): No such file or
directory
Attaching 1 probe...
sleep nanoslept
splunkd nanoslept
webrtc_audio_mo nanoslept
gnome-terminal- nanoslept
webrtc_audio_mo nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept

$
```

Ambigous executable:
```
$ sudo ./build/src/bpftrace -e 'tracepoint:syscalls:sys_enter_nanosleep
{ printf("%s nanoslept\n", comm); }' -c 'sleep 1'
chdir(/lib/modules/4.19.8-200.fc28.x86_64/build): No such file or
directory
Attaching 1 probe...
execve: No such file or directory
Failed to spawn child=sleep 1
splunkd nanoslept

$
```

This closes #253