This patch allows the raw driver to be built as a kernel module.

It also cleans up a bunch of stuff, C99ifies the initialisers, gives
lots of symbols static scope, etc.

The module is unloadable when there are zero bindings.  The current
ioctl() interface have no way of undoing a binding - it only allows
bindings to be overwritten.  So I overloaded a bind to major=0,minor=0
to mean "undo the binding".  I'll update the raw(8) manpage for that.

generic_file_direct_IO has been exported to modules.

The call to invalidate_inode_pages2() has been removed from all
generic_file_driect_IO() callers, into generic_file_direct_IO() itself.
Mainly to avoid exporting invalidate_inode_pages2() to modules.