fix missing/inet_ntop.c again

All tests that should print IPv6-addresses failed since 'INET6' wasn't
set when my missing/inet_ntop.c was compiled. Due to "config.h" was not
included.

Also got rid of 2 warnings:
 Missing/inet_ntop.c:146:23: warning: 'cur.len' may be used
 uninitialized in this function [-Wmaybe-uninitialized]

The tests also failed since the inet_ntop_v6() was returning hex-chars
in upper-case. So this patch returns string in lower-case.