cryptsetup: modify keyfile search logic to use read_file_full() too

cryptsetup: modify keyfile search logic to use read_file_full() too

Let's move the 3rd way how cryptsetup acquires key files to
read_file_full() too.

Since load_key_file()'s raison d'etre now is just the search path logic,
let's rename the function to find_key_file().