file-systems: Add 'find-partition-by-luks-uuid'.

* gnu/build/file-systems.scm (%luks-endianness, %luks-header-size): New
macros.
(%luks-magic): New variable.
(sub-bytevector, read-luks-header, luks-header-uuid): New procedures.
(partition-predicate): Add 'read' parameter; wrap it with 'ENOENT-safe'.
Use it instead of 'read-ext2-superblock*'.
(read-ext2-superblock*): Remove.
(partition-label-predicate, partition-uuid-predicate): Pass
'read-ext2-superblock' as the first argument.
(partition-luks-uuid-predicate): New variable.
(find-partition-by-luks-uuid): New procedure.