bootspec: try harder to suppress duplicate enumerated entries

bootspec: try harder to suppress duplicate enumerated entries

For testing purposes I run one of my system symlinking /boot/loader/ to
/efi/loader/. This triggers annoying behaviour in boot entry
enumeration: the code ends up iterating through the 'entries' subdir of
both, thinking one was actually in the ESP and the other in XBOOTLDR,
and thus distinct. This would result in duplicate entries.

Let's address that, and filter out duplicates via their inode numbers:
never process the same inode twice. This should protect us from any
confusion effectively, regardless which inodes are symlinked (or even
bind mounted).