signal: remove window for SignalException on IGNORE

* signal.c (signal_exec): ignore immediate cmd for SIG_IGN
* signal.c (trap_handler): set cmd to true for SIG_IGN
* signal.c (trap): handle nil and true values for oldcmd
  [Bug #9835]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46194 b2dd03c8-39d4-4d8f-98ff-823fe69b080e