In the SUPPORT_DOT case, warn that seperation with a period is deprecated.

In -STABLE, this is default, in -CURRENT it is not, which leads to many a
headache for a user coming to -CURRENT without remembering this fact.  It
is one of the POLA violations we have not avoided by preparing the users
for it appopriately.  Therefore, a warnx(3) is added here, explicitly to
be MFC'd shortly to start the re-education process rolling.

Reviewed by:	General murmurs of approval in that IRC channel.
MFC after:	3 days

usr.sbin/chown/chown.c

@@ -145,6 +145,7 @@ main(argc, argv)
 		}
 #ifdef SUPPORT_DOT
 		else if ((cp = strchr(*argv, '.')) != NULL) {
+			warnx("seperation of user and group with a period is deprecated");
 			*cp++ = '\0';
 			a_gid(cp);
 		}