session.h (struct systemtap_session): Added itrace_derived_probe

session.h (struct systemtap_session): Added itrace_derived_probe
group.
elaborate.cxx (systemtap_session::systemtap_session): Added
initialization of itrace_derived_probes.
tapsets.cxx (struct itrace_derived_probe): Add derived_probe
struct for holding info needed by itrace probes.
(struct itrace_derived_probe_group): New derived_probe_group
to handle itrace probes.
(itrace_derived_probe::itrace_derived_probe): Needed for use with
task_finder.
(itrace_derived_probe_group::join_group): Ditto.
(itrace_derived_probe_group::enroll): Ditto.
(itrace_derived_probe_group::emit_probe_decl): Ditto.
(itrace_derived_probe_group::emit_module_decls): Ditto.
(itrace_derived_probe_group::emit_module_init): Ditto.
(itrace_derived_probe_group::emit_module_exit): Ditto.
stapprobes.5.in : Added documentation of itrace probe.