shared: rename crypt-util.c → cryptsetup-util.c

"crypt-util.c" is such a generic name, let's avoid that, in particular
as libc's/libcrypt's crypt() function is so generically named too that
one might thing this is about that. Let's hence be more precise, and
make clear that this is about cryptsetup, and nothing else.

We already had cryptsetup-util.[ch] in src/cryptsetup/ doing keyfile
management. To avoid the needless confusion, let's rename that file to
cryptsetup-keyfile.[ch].