Similar to r130943 for cdboot.S, update the license on this file to a
stock 2-clause BSD license.

MFC after:	1 week
- Update a few places to account for va_copy().
- Create a separate 'return values' section and move some statements about
  return values to that section.
- Note that each invocation of va_start() and va_copy() must be paired with
  va_end() in the same function.

MFC after:	1 week