CRYPTO: Use unprefixed version of function stpncpy
    
    glibc contains two versions of finction stpncpy
    sh-4.2$ nm --dynamic --defined-only /lib64/libc.so.6 | grep stpncpy
    0000003ce1c89b00 i stpncpy
    0000003ce1c89b00 i __stpncpy
    0000003ce1d040a0 T __stpncpy_chk
    
    We should use more portable version of stpncpy without prefix with underscores.
    
    The function __stpncpy was used in initial veersion
    e65c65fc710fa030bfb8319efc43fcdc9ce5a26f, which was based on
    http://people.redhat.com/drepper/SHA-crypt.txt
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 83e1ea9e24aabe6ade180649ecfc9c253ad19395)