Implement two suggestions by Jonathan Giddy: (1) in AIX, clear the

Implement two suggestions by Jonathan Giddy: (1) in AIX, clear the
data struct before calling gethostby{name,addr}_r(); (2) ignore the
3/5/6 args determinations made by the configure script and switch on
platform identifiers instead:

AIX, OSF have 3 args
Sun, SGI have 5 args
Linux has 6 args

On all other platforms, undef HAVE_GETHOSTBYNAME_R altogether.