feat(eficore): find the extended boot loader partition on the disk of a partition

Look for a filesystem on the same disk whose GPT partition type is the
XBOOTLDR type, using the partition info protocol. open_shared moves to
the handle module so the graphical menu and the discovery share it.

crates/eficore/src/handle.rs

@@ -1,5 +1,6 @@
 use anyhow::{Context, Result};
-use uefi::boot::SearchType;
+use uefi::boot::{OpenProtocolAttributes, OpenProtocolParams, ScopedProtocol, SearchType};
+use uefi::proto::ProtocolPointer;
 use uefi::{Guid, Handle};
 use uefi_raw::Status;
 
@@ -24,3 +25,21 @@ pub fn find_handle(protocol: &Guid) -> Result<Option<Handle>> {
         }
     }
 }
+
+/// Open the protocol `P` on `handle` without taking it from the firmware.
+/// An exclusive open makes the firmware disconnect every driver that is using the protocol,
+/// which for a disk or a partition would remove the filesystem Sprout is running from.
+pub fn open_shared<P: ProtocolPointer + ?Sized>(handle: Handle) -> uefi::Result<ScopedProtocol<P>> {
+    // SAFETY: The protocols opened this way are only used from this thread, and the firmware
+    // keeps them installed for as long as they are open.
+    unsafe {
+        uefi::boot::open_protocol::<P>(
+            OpenProtocolParams {
+                handle,
+                agent: uefi::boot::image_handle(),
+                controller: None,
+            },
+            OpenProtocolAttributes::GetProtocol,
+        )
+    }
+}