stap: Add shorthand option --dyninst for --runtime=dyninst

stap: Add shorthand option --dyninst for --runtime=dyninst

* cmdline.cxx: Define --dyninst as LONG_OPT_RUNTIME_DYNINST.
* session.cxx (systemtap_session::parse_cmdline_runtime): Pull out the
  code to set the runtime option.
  (systemtap_session::parse_cmdline): Use parse_cmdline_runtime for both
  LONG_OPT_RUNTIME and LONG_OPT_RUNTIME_DYNINST.
  (systemtap_session::usage): Document --dyninst.
* man/stap.1: Document --dyninst.