We no longer include <netinet/ip.h>, so we don't need to work around

We no longer include <netinet/ip.h>, so we don't need to work around
Digital UNIX using "ip_vhl" rather than "ip_v" and "ip_hl" bitfields if
__STDC__ is defined (we have our own "ip.h", and we don't use bitfields
in it).

Get rid of HAVE_INET_ATON, HAVE_INET_NTOP, and HAVE_INET_PTON - we don't
use them; instead, we supply our own versions of "inet_aton()",
"inet_ntop()", and "inet_pton()".  (Defining them in "configure.in"
caused complaints from "autoreconf".)

Check for the "rc5.h" header, as we *do* use HAVE_RC5_H.