uefi-raw: Fill in [un]install_multiple_protocol_interfaces pointers

The existing comment there said the unstable `c-variadic` feature was required,
but that was incorrect. While defining c-variadic functions is unstable in Rust,
c-variadic function pointers are stable (calling c-variadics is also stable).

We still can't quite define these two function pointers correctly because they
should be `extern "efiapi", which requires the unstable
`extended_varargs_abi_support` feature. However, as long as your code is
compiled for a UEFI target, `extern "C"` gives you the correct ABI. Since these
function pointers are `unsafe` to call, we can expose them in the raw interface
and leave it up to the caller to ensure they are used correctly.

Author: nicholasbishop

uefi-raw/CHANGELOG.md

@@ -6,3 +6,7 @@
 
 ## Changed
 - `{install,reinstall,uninstall}_protocol_interface` now take `const` interface pointers.
+- `{un}install_multiple_protocol_interfaces` are now defined as c-variadic
+  function pointers. The ABI is `extern "C"` until such time as
+  [`extended_varargs_abi_support`](https://github.com/rust-lang/rust/issues/100189)
+  is stabilized.

uefi-raw/src/table/boot.rs

@@ -188,11 +188,28 @@ pub struct BootServices {
         out_proto: *mut *mut c_void,
     ) -> Status,
 
-    // These two function pointers require the `c_variadic` feature, which is
-    // not yet available in stable Rust:
-    // https://github.com/rust-lang/rust/issues/44930
-    pub install_multiple_protocol_interfaces: usize,
-    pub uninstall_multiple_protocol_interfaces: usize,
+    /// Warning: this function pointer is declared as `extern "C"` rather than
+    /// `extern "efiapi". That means it will work correctly when called from a
+    /// UEFI target (`*-unknown-uefi`), but will not work when called from a
+    /// target with a different calling convention such as
+    /// `x86_64-unknown-linux-gnu`.
+    ///
+    /// Support for C-variadics with `efiapi` requires the unstable
+    /// [`extended_varargs_abi_support`](https://github.com/rust-lang/rust/issues/100189)
+    /// feature.
+    pub install_multiple_protocol_interfaces:
+        unsafe extern "C" fn(handle: *mut Handle, ...) -> Status,
+
+    /// Warning: this function pointer is declared as `extern "C"` rather than
+    /// `extern "efiapi". That means it will work correctly when called from a
+    /// UEFI target (`*-unknown-uefi`), but will not work when called from a
+    /// target with a different calling convention such as
+    /// `x86_64-unknown-linux-gnu`.
+    ///
+    /// Support for C-variadics with `efiapi` requires the unstable
+    /// [`extended_varargs_abi_support`](https://github.com/rust-lang/rust/issues/100189)
+    /// feature.
+    pub uninstall_multiple_protocol_interfaces: unsafe extern "C" fn(handle: Handle, ...) -> Status,
 
     // CRC services
     pub calculate_crc32: