cryptsetup: port cryptsetup's main key file logic over to read_full_file_full()

cryptsetup: port cryptsetup's main key file logic over to read_full_file_full()

Previously, we'd load the file with libcryptsetup's calls. Let's do that
in our own, so that we can make use of READ_FULL_FILE_CONNECT_SOCKET,
i.e. read in keys via AF_UNIX sockets, so that people can plug key
providers into our logic.

This provides functionality similar to Debian's keyscript= crypttab
option (see → #3007), as it allows key scripts to be run as socket
activated services, that have stdout connected to the activated socket.
In contrast to traditional keyscript= support this logic runs stuff out
of process however, which is beneficial, since it allows sandboxing and
similar.