tmux(1): remove -d option from optstring

[Theo Buehler]

The -d option in tmux was removed five years ago in revision 1.52 of
tmux.c.  Also remove it from the optstring argument to getopt():

Index: tmux.c
===================================================================
RCS file: /cvs/src/usr.bin/tmux/tmux.c,v
retrieving revision 1.132
diff -u -p -r1.132 tmux.c
--- tmux.c      20 Oct 2014 23:27:14 -0000      1.132
+++ tmux.c      10 Nov 2014 11:20:07 -0000
@@ -218,7 +218,7 @@ main(int argc, char **argv)
        flags = 0;
        label = path = NULL;
        login_shell = (**argv == '-');
-       while ((opt = getopt(argc, argv, "2c:Cdf:lL:qS:uUv")) != -1) {
+       while ((opt = getopt(argc, argv, "2c:Cf:lL:qS:uUv")) != -1) {
                switch (opt) {
                case '2':
                        flags |= CLIENT_256COLOURS;