* tapset/context.stp: Add function target().
* stapfuncs.5.in (target): Document it.
* elaborate.h (struct systemtap_session): Add cmd and target_pid to
the struct.
* main.cxx (usage): Add secriptions of "-c" and "-x" options.
(main): Set s.cmd and s.target_pid.
* buildrun.cxx (stringify): Copy this utility func here too.
(run_pass): Add new options to set cmd and pid to the stpd
command line.