Make srvx compile on cygwin again.

configure.in (AC_CHECK_HEADERS): Look for <arpa/inet.h>.
  (AC_CHECK_FUNCS): Look for inet_aton().

src/compat.c: Try to include <arpa/inet.h> so we get inet_addr().
  (getaddrinfo): If inet_aton() is not provided, use inet_addr() instead.

src/compat.h (AI_NUMERICHOST): Define this too.

src/tools.c (irc_pton): Use the pedantically correct type for ip4.
git-archimport-id: srvx@srvx.net--2006/srvx--devo--1.3--patch-30