fsck_msdosfs(8): Introduce -B option as no-op

When performing a background fsck on msdosfs devices, it ends
with the following error:

fsck_msdosfs: illegal option -- B
usage: fsck_msdosfs -p [-f] filesystem ...
       fsck_msdosfs [-ny] filesystem ...

Introduce -B option as a compatibility with fsck_ffs(8) and
also update the descriptions for -B and -C in fsck_msdosfs(8)
manual page.

Reviewed by:	imp
Approved by:	imp
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D48132

sbin/fsck_msdosfs/fsck_msdosfs.8

@@ -23,7 +23,7 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd January 6, 2020
+.Dd December 18, 2024
 .Dt FSCK_MSDOSFS 8
 .Os
 .Sh NAME
@@ -32,10 +32,10 @@
 .Sh SYNOPSIS
 .Nm
 .Fl p
-.Op Fl Cf
+.Op Fl BCf
 .Ar filesystem ...
 .Nm
-.Op Fl CMny
+.Op Fl BCMny
 .Ar filesystem ...
 .Sh DESCRIPTION
 The
@@ -71,10 +71,14 @@ making any changes.
 .Pp
 The options are as follows:
 .Bl -tag -width indent
-.It Fl C
-Compatibility with the corresponding
+.It Fl B
+Ignored for
 .Xr fsck 8
-option (skip check if clean), defined to no-op.
+compatibility.
+.It Fl C
+Ignored for
+.Xr fsck 8
+compatibility.
 .It Fl F
 Compatibility with the wrapper
 .Xr fsck 8

sbin/fsck_msdosfs/main.c

@@ -68,9 +68,10 @@ main(int argc, char **argv)
 
 	skipclean = 1;
 	allow_mmap = 1;
-	while ((ch = getopt(argc, argv, "CfFnpyM")) != -1) {
+	while ((ch = getopt(argc, argv, "BCfFnpyM")) != -1) {
 		switch (ch) {
-		case 'C': /* for fsck_ffs compatibility */
+		case 'B': /* for fsck_ffs compatibility */
+		case 'C':
 			break;
 		case 'f':
 			skipclean = 0;