util/crypto: generate_csprng_buffer() changed
1) generate_csprng_buffer() is renamed to sss_generate_csprng_buffer()
to make util/crypto API more consistent
2) its implementation became dependant on crypto backend being used
3) in case of libcrypto backend RAND_bytes() is used instead of
direct access to "/dev/urandom"
Relates: https://pagure.io/SSSD/sssd/issue/4024
Reviewed-by: Pavel Březina <pbrezina@redhat.com>