Fix ./configure EUI check (#2338)

Fix missing header `sys/socket.h`:

    configure:29217: checking for struct arpreq.arp_pa
    .../net/if_arp.h: error: field has incomplete type 'struct sockaddr'

Fix `ac_aggr.arp_pa` test, replacing it with a test of a data member
that Squid code is actually using:

    conftest.cpp:112:5: error: value of type 'struct sockaddr' is not
        contextually convertible to 'bool'

Discovered while porting Squid v7 to FreeBSD.

Author: timp87

configure.ac

@@ -991,7 +991,8 @@ include <windows.h>
 #include <sys/param.h>
 #endif
   ]])
-  AC_CHECK_MEMBER([struct arpreq.arp_pa],[],[
+  AC_CHECK_HEADERS(sys/socket.h)
+  AC_CHECK_MEMBER([struct arpreq.arp_ha.sa_family],[],[
     AS_IF([test "x$squid_host_os" != "xmingw" -a "x$squid_host_os" != "xcygwin"],[
       AC_MSG_NOTICE([OS support for EUI seems incomplete, disabling it])
       enable_eui=no