Exit rather than panic for most errors.

In the FreeBSD UEFI boot protocol, boot1.efi exits back to UEFI if it
can't boot the image for most reasons (so that further items in the
EFI boot manger list can be tried). Rename panic to efi_panic, make it
static and give it an extra status argument. Exit back to UEFI with
that status argument so the next loader can be tried.

Use malloc/free exclusively instead of mixing malloc/free and
AllocatePool/FreePool. The code is smaller.

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