lib/util.c: Add yescrypt hashing method for user passwords.
    
    The yescrypt hashing method is considered to be much stronger than
    sha512crypt and fully supported by libxcrypt >= 4.3.  It is based
    on NIST-approved primitives and on par with argon2 in strength.
    
    Signed-off-by: Björn Esser <besser82@fedoraproject.org>