Add process("NAME") to perf probe.

Add process("NAME") to perf probe.

* NEWS: Added.
* stapprobes.3stap: Added.

* perf.c (_stp_perf_init,_stp_perf_del):  Add support for task.

* perf.h (stap_perf_probe):  Add per_thread, per_thread_event, tgt.

* tapset-perfmon.cxx (TOK_PROCESS): New
  (perf_derived_probe): Add has_process, process_name.
  (join_group): Enable the task finder.
  (emit_module_decls): Add perf_event.h.  perf.h is needed sooner.
  Emit task finder callback: _stp_perf_probe_cb.  Setup task finder.

* perf.exp, towers.c: New.
* unprivileged_probes.exp (restricted_probe_types): Added
  perf.type(number).config(number).process