Fixes and updates to signal handling

* Signal.trap now always returns the previous handler for the trapped
  signal, or the string DEFAULT or SYSTEM_DEFAULT as appropriate. This
  is the same behavior as MRI.
* Signal 0 (SYSEXIT) in MRI is now implemented. If a SYSEXIT trap is
  set, it will be executed as the JRuby runtime is torn down.
* Added `Signal.sigame(0)` support.
* Added taint checking for trap handlers, for MRI parity
* Added reserved signal checking for MRI parity
* Fixed a bug that previously prevented IGNORE from being sent as a trap
* Un-excluded a number of Signal tests