Misc Selector improvements.

* Remove sel_registerName, it's identical to GetHandle. Change
  all usages of sel_registerName to use GetHandle instead.

* There's no need to leak in the ctor. sel_registerName will copy
  the input string if needed (documentation doesn't mention this, but
  the source code at www.opensource.apple.com confirms it).

* Simplify the comparison operators, a simple == operation is enough,
  both according to documentation and www.opensource.apple.com.