Add a deprecation warning when using the feature which mounts devices

to see how much space it on them.

Adjust MOUNT_CHAR_DEVS to allow the free space of already mounted
devices to be displayed and report an appropriate error if the
device isn't mounted.

Reviewed by:	cem
MFC after:	1 week
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D8801

bin/df/df.c

@@ -254,9 +254,11 @@ main(int argc, char *argv[])
 				rv = 1;
 				continue;
 			}
-#ifdef MOUNT_CHAR_DEVS
 		} else if (S_ISCHR(stbuf.st_mode)) {
 			if ((mntpt = getmntpt(*argv)) == NULL) {
+#ifdef MOUNT_CHAR_DEVS
+				xo_warnx(
+				    "df on unmounted devices is deprecated");
 				mdev.fspec = *argv;
 				mntpath = strdup("/tmp/df.XXXXXX");
 				if (mntpath == NULL) {
@@ -305,8 +307,12 @@ main(int argc, char *argv[])
 				(void)rmdir(mntpt);
 				free(mntpath);
 				continue;
-			}
+#else
+				xo_warnx("%s: not mounted", *argv);
+				rv = 1;
+				continue;
 #endif
+			}
 		} else
 			mntpt = *argv;