Revision of Marcin's QError module

* I think, there was a rough consensus that Marcin's module provides the
  right kind of interface for handling "errno"s

* Malcolm preferred an enumeration, but Marcin made a convincing argument
  that this doesn't go well with architecture-dependent variations of "errno"

* Recoded the hsc magic into autoconf magic

* In hsc files, errno constants can be accessed directly; to provide access
  for ordinary Haskell programmer, constants for a range of common symbols are
  provided (their values are determined by configure)

* Introduced an `Errno' newtype to increase type safety

* renamed the throwIfXXX functions to throwErrnoIfXXX to have the original
  names available in the language independent MarshalError

* FIXME: We need a thread local getErrno in the RTS!

(It all compiles, but is still to be tested.)