Raw.xs: allow resolving functions in main program
Raw.xs: use Windows style interfaces on Cygwin

The Unixy dl* interfaces are actually just wrappers around the
windows interfaces on Cygwin anyway.  For finer grain control
in windows use the windows interface
Raw.xs: emulate dlopen(NULL, ...) on Windows

Passing in NULL as the library name to dlopen allows you to search
the current executable and its loaded dynamic libraries.  There is
no equivalent to this in Windows, so emulate the behavior using
psapi.
Changes: update
Raw.pm: document $library = undef behaviour