[Bug tapsets/13330] Support TRACE_SYSTEM for tracepoints

[Frank Ch. Eigler]

> --- Comment #2 from Jonathan Lebon <jlebon at redhat dot com> ---
> Added support for TRACE_SYSTEM in branch jlebon/trace_system:
> [...]

Looked the branch over briefly.  The work looks sound and useful,
thank you!

(As an OOPS style nit, presence of both a public member variable and a
public member function to compute its initial/sole value, is a bit
odd. (tracepoint_query::current_system and ::retrieve_*).  The
codebase is not exactly OOPS-pure, but if we can move toward it
slowly, that'd be good.  e.g., initialize the variable during a ctor,
or from a private: function, or ... something.  No big deal though.

- FChE