Make -t the default on FreeBSD to match the current behavior there.  Add
a -T to disable this behavior.

(r240792 fixed a typo in the docs that I fixed by hand below.)

Index: mtree.8
===================================================================
--- mtree.8	(revision 240789)
+++ mtree.8	(revision 240790)
@@ -300,6 +300,16 @@
 .It Fl t
 Modify the modified time of existing files, the device type of devices, and
 symbolic link targets, to match the specification.
+This is the default on
+.Fx
+systems.
+.It Fl T
+.Em Do not
+modify the modified time of existing files, the device type of devices, and
+symbolic link targets, to match the specification.
+This is the default on systems other than
+.Fx
+.
 .It Fl U
 Same as
 .Fl u
Index: mtree.c
===================================================================
--- mtree.c	(revision 240789)
+++ mtree.c	(revision 240790)
@@ -79,8 +79,12 @@
 	spec1 = stdin;
 	spec2 = NULL;
 
+#ifdef __FreeBSD__
+	tflag = 1;
+#endif
+
 	while ((ch = getopt(argc, argv,
-	    "cCdDeE:f:I:ijk:K:lLmMnN:p:PqrR:s:StuUwWxX:"))
+	    "cCdDeE:f:I:ijk:K:lLmMnN:p:PqrR:s:StTuUwWxX:"))
 	    != -1) {
 		switch((char)ch) {
 		case 'c':
@@ -187,6 +191,9 @@
 		case 't':
 			tflag = 1;
 			break;
+		case 'T':
+			tflag = 0;
+			break;
 		case 'u':
 			uflag = 1;
 			break;