The exception raised in the BPF class constructor assumed
that the "src_file" argument was used to pass eBPF code -
this is not the case for many of the tools scripts, which
tend to use "text".

Signed-off-by: Nathan Scott <[email protected]>

Several python tools allow their eBPF code to be printed to
stdout for debugging.  There are other projects that would
like to share these program definitions however, instead of
duplicating code.  Formalise an -e/--ebpf option, and start
using it in several tools (more to come).

Signed-off-by: Nathan Scott <[email protected]>

As discussed in iovisor#1531
review comments.

Signed-off-by: Nathan Scott <[email protected]>

Use of this argparse constructor keyword is causing regression
test failures, and its use was nice-to-have - this means the -e
shorthand for --ebpf will be available iff its not been used in
another add_argument call.  Neither -e/--ebpf are advertised in
the usage message anyway.