New DLL and Proc types to manage and call dll functions. These were
used to simplify syscall tests in runtime package. They were also
used to implement LazyDLL and LazyProc.

LazyProc, like Proc, now have Call function, that just a wrapper for
SyscallN. It is not as efficient as Syscall, but easier to use.

NewLazyDLL now supports non-ascii filenames.

LazyDLL and LazyProc now have Load and Find methods. These can be used
during runtime to discover if some dll functions are not present.

All dll functions now return errors that fit os.Error interface. They
also contain Windows error number.

Some of these changes are suggested by jp.

R=golang-dev, jp, rsc
CC=golang-dev
https://golang.org/cl/5272042