Fix libargon2 in debian-installer

Debian-installer doesn't ship with everything required for pthreads support, and libargon2 defaults to using threads. This breaks cryptsetup when trying to create partitions using argon2 as a kdf. The current version of argon2 supports building without any threading support, so this patchset updates to the current version and builds the udeb separately. Unfortunately this introduces a SONAME bump, but there's no actual ABI change so we can ship a couple of compatibility packages.