[efi] Allow compiler to perform type checks on EFI_HANDLE

The EFI headers define EFI_HANDLE as a void pointer, which renders
type checking on anything dealing with EFI handles somewhat useless.
Work around this bizarre sabotage attempt by redefining EFI_HANDLE as
a pointer to an anonymous structure.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Avoid unnecessarily passing pointers to EFI_HANDLEs

efi_file_install() and efi_download_install() are both used to install
onto existing handles.  There is therefore no need to allow for each
of their calls to InstallMultipleProtocolInterfaces() to create a new
handle.

By passing the handle directly (rather than a pointer to the handle),
we avoid potential confusion (and erroneous debug message colours).

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Dump existing openers when we are unable to open a protocol

Dump the existing openers of a protocol whenever we are unable to open
a protocol using attributes of BY_DEVICE, EXCLUSIVE, or
BY_CHILD_CONTROLLER.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Dump handle information around connect/disconnect attempts

Signed-off-by: Michael Brown <mcb30@ipxe.org>