menu: Stop the persistent EFI variables overriding default_entry

[wehrwolfmann]

Fixes #652.

On a dual-boot box default_entry stopped preselecting Windows: whatever was
booted last came up instead, so the machine booted Linux on the timeout.

The selection in _menu is a chain of if (!has_entry) steps.
LoaderEntryOneShot sets has_entry once it resolves an entry. The
default_entry block right below it sets selected_entry and leaves
has_entry alone - not in the index branch, not in the path branch. So the
next two blocks still see it false: remember_last_entry reads
LimineLastBootedEntry and overwrites the selection, and
bli_get_default_entry shadows it the same way. A numeric default_entry
goes through the same block, so switching to an index does not help either.

CONFIG.md documents the opposite, so this is a code bug and not a doc one:

remember_last_entry and the persistent LoaderEntryDefault variable are
consulted, in that order, only where this option is not set.

The patch sets has_entry in both branches, the way LoaderEntryOneShot
already does. The path branch only sets it once the path resolved, so an
unresolvable default_entry keeps falling through as before.

Built clean on trunk with ./configure --enable-uefi-x86-64 CC_FOR_TARGET=clang, and again with --enable-bios since the block is not
UEFI-only; no new warnings. On the affected machine remember_last_entry: no
brings the preselection back, which is the workaround the issue describes.