RFC: systemtap remote shell (stapsh)

[Josh Stone]

Reproducing some feedback given on freenode #systemtap:

>> //   command: arg SIZE
>> //            DATA
>> //     reply: (none)
>> //      desc: Push an argument of SIZE bytes for staprun.  Note that while it
>> //            is read as binary data, any embedded NIL will truncate the
>> //            argument in the actual command invocation.
>> //
>> //   command: run
>> //     reply: (none)
>> //      desc: Start staprun with the previously pushed arguments.  When the
>> //            child exits, stapsh will clean up and then exit with the same
>> //            return code.

fche suggested these should be combined to just "run ARG1 ARG2 ...",
where each ARG is encoded as perhaps base64 or quoted-printable.

>> //   command: signal NUM
>> //     reply: (none)
>> //      desc: Send signal NUM to the child process.

przemoc pointed out that signal numbers are architecture-specific, so
this should perhaps use names instead.  At the moment, I'm only actually
using SIGINT, which appears standardized at signal #2, but changing to
names is probably still a good idea.