ifconfig(8) to deny non-contiguous netmask / take 2

[Denis Fondras]

Rework the previous diff. Thanks to job@ who noticed that it failed in some
cases.

Index: ifconfig.c
===================================================================
RCS file: /cvs/src/sbin/ifconfig/ifconfig.c,v
retrieving revision 1.378
diff -u -p -r1.378 ifconfig.c
--- ifconfig.c  9 Sep 2018 20:32:55 -0000       1.378
+++ ifconfig.c  30 Sep 2018 07:42:26 -0000
@@ -5760,6 +5760,9 @@ in_getaddr(const char *s, int which)
                else
                        errx(1, "%s: bad value", s);
        }
+       if (which == MASK && (ntohl(sin->sin_addr.s_addr) &
+           (~ntohl(sin->sin_addr.s_addr) >> 1)))
+               errx(1, "%s: non-contiguous mask", s);
 }
 
 /* ARGSUSED */