add system() call

attach to multiple identical probes

There were two problems keeping bpftrace from attaching to multiple
identical probes in the same script: the first one was in our IR code
generation, where we were creating multiple sections with the same name
(since the secion name is determined by the probe's name). Since
sections must have unique names, LLVM was not able to translate our IR
to BPF. This was resolved by appending a `_#` to the section's name.

The second problem was attaching to that probe: we can't use the same
event name multiple times in "kprobe_events". bcc already handles
multiple probes coming from *different* processes, since it also
includes the PID on the event name. The solution was similar, we append
`_#` to the event name before sending it to bcc. This required some
strucutral changes, like keeping the `#` (index) in the AST Probe nodes
and calling `BPFTrace::add_probe` during codegen instead of during
semantic analysis (otherwise we wouldn't have the index for our Probes).

Fixes: https://github.com/iovisor/bpftrace/issues/97

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