sprintf is still used in Kea code

Here is the error given by last macOS Xcode:

fqdn_unittest.cc:637:9: error: 'sprintf' is deprecated: This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead. [-Werror,-Wdeprecated-declarations]
        sprintf(config_buf, config_template, mode);
        ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/stdio.h:188:1: note: 'sprintf' has been explicitly marked deprecated here
__deprecated_msg("This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead.")
^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/sys/cdefs.h:215:48: note: expanded from macro '__deprecated_msg'
        #define __deprecated_msg(_msg) __attribute__((__deprecated__(_msg)))
                                                      ^
1 error generated.
make[6]: *** [dhcp4_unittests-fqdn_unittest.o] Error 1

We should remove it ASAP as sprintf is really a security issue.