Sort of "fixed" KNOWN_BUGS #4: curl now builds IPv6 enabled on AIX 4.3. At

least it should no longer cause a compiler error. However, it does not have
AI_NUMERICHOST so we cannot getaddrinfo() any numerical addresses with it (we
use that for FTP PORT/EPRT)! So, I modified the configure check that checks if
the getaddrinfo() is working, to use AI_NUMERICHOST since then it'll fail on
AIX 4.3 and it will automatically build with IPv6 support disabled.