tree-wide: use READ_FULL_FILE_CONNECT_SOCKET at various places

tree-wide: use READ_FULL_FILE_CONNECT_SOCKET at various places

Let's use the new flag wherever we read key material/passphrases/hashes
off disk, so that people can plug in their own IPC service as backend if
they like, easily.

(My main goal was actually to support this for crypttab key files — i.e.
that you can specify AF_UNIX sockets as third column in crypttab — but
that's harder to implement, since the keys are read via libcryptsetup's
API, not ours.)