Always use get{addr|name}info() when available

Both getaddrinfo() and getnameinfo() are now used always when available, and
not only when compiling ngIRCd with support for IPv6.

This not only enables ngIRCd to handle multiple addresses per hostname when
compiled without support for IPv6, but fixes binding ngIRCd to IP addresses
on Mac OS X (and probably other BSD-based systems) as well: these systems
require that sockaddr_in is zeroed out and sockaddr_in.sin_len is set to
sizeof(sockaddr_in) like that:

  src/ipaddr/ng_ipaddr.c, line 54:

        assert(ip_str);
      + memset(addr, 0, sizeof *addr);
      + addr->sin4.sin_len = sizeof(addr->sin4);
        addr->sin4.sin_family = AF_INET;

But this would break all the systems not using sockaddr_in.sin_len, for
example Linux -- so we assume that all these systems provide getaddrinfo()
and use that for now.

configure.in

@@ -73,7 +73,6 @@ AC_DEFUN([GCC_STACK_PROTECT_CC],[
   fi
 ])
 
-
 if test "$GCC" = "yes"; then
 	# We are using the GNU C compiler. Good!
 	CFLAGS="$CFLAGS -pipe -W -Wall -Wpointer-arith -Wstrict-prototypes"
@@ -125,7 +124,6 @@ AC_TRY_COMPILE([
 AC_TYPE_SIGNAL
 AC_TYPE_SIZE_T
 
-
 # -- Libraries --
 
 AC_CHECK_LIB(UTIL,memmove)
@@ -141,7 +139,8 @@ AC_CHECK_FUNCS([ \
 	bind gethostbyaddr gethostbyname gethostname inet_ntoa \
 	setsid setsockopt socket strcasecmp waitpid],,AC_MSG_ERROR([required function missing!]))
 
-AC_CHECK_FUNCS(inet_aton isdigit sigaction snprintf vsnprintf strdup strlcpy strlcat strtok_r)
+AC_CHECK_FUNCS(getaddrinfo getnameinfo inet_aton isdigit sigaction snprintf \
+ vsnprintf strdup strlcpy strlcat strtok_r)
 
 # -- Configuration options --
 
@@ -479,6 +478,8 @@ AC_ARG_ENABLE(ipv6,
 	if test "$enableval" = "yes"; then x_ipv6_on=yes; fi
 )
 if test "$x_ipv6_on" = "yes"; then
+	# getaddrinfo() and getnameinfo() are optional when not compiling
+	# with IPv6 support, but are required for IPv6 to work!
 	AC_CHECK_FUNCS([ \
 		getaddrinfo getnameinfo \
 		],,AC_MSG_ERROR([required function missing for IPv6 support!]))