PR11004: try / catch error-handling script syntax

PR11004: try / catch error-handling script syntax

* parse.h (try_block): New class.  Update basic visitors.
* staptree.cxx: Implement basic visitors.
* parse.cxx (expect_kw): Fix to actually look for keywords.
  (parse_try_block): New function.
  (lexer ctor): Designate 'try' and 'catch' as keywords.
* elaborate.cxx (dead_assignment_remover, dead_statmtexpr_remover): Optimize.
  (other visitors): Implement.
* translate.cxx (c_unparser): Implement via super-handy __local__ labels.
  (emit_probe, emit_function): Make outer out: label also __local__.
* testsuite/buildok/fortyone.stp, semko/fortynine.stp,
  systemtap.base/trycatch.exp: Test it.
* NEWS, doc/langref.txt, stap.1.in: Document it.