Fix to handle properly 'catch signal SIGINT' and SIGTRAP
* spawn.cc (ILLEGAL_SIG_FUNC_PTR): New define.
(system_call_handle): Rename from system_call_cleanup.
(is_system_call): New convenience method.
(system_call_handle::system_call_handle): Use ILLEGAL_SIG_FUNC_PTR rather than
cast.  Call sig_send here rather than in caller.  Initialize oldint.
(system_call_handle::arm): New function pulled from constructor.
(~system_call_handle::system_call_handle): Use is_system_call().
(child_info_spawn::worker): Use system_call_handle to set up for system call
early.  Use arm call prior to waiting for child to properly set up signal
handling.  Move comment closer to code it is commenting on.