signal.c: raise SystemCallError for all failures

* signal.c (ruby_signal): return SIG_ERR as well as signal(2).

* signal.c (trap): raise SystemCallError for all failures when
  called as a method.

* signal.c (Init_signal): fail by [BUG] only if initialization is
  failed.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47660 b2dd03c8-39d4-4d8f-98ff-823fe69b080e