Fix cleanup of BPF unmount

[mariomac]

This fixes some aspects of BPF map unmounting:

• Often, the os.Exit function in the shutdown hook was invoked before the unmounting code, so the PID folders remained unmounted.
• Removed the os.Exit invocation sometimes caused the system to panic with "internal error: exit hook invoked exit".
  • To avoid the process to be blocked if it did not found the target executable, we added a context to the InspectOffsets function that stops searching if the context is canceled.
• The mount/unmount code has been moved from main.go to the eBPF package, as it is actually a functionality related to BPF.

[grcevski]

LGTM! This looks great!