makefs: Make it possible to silence warnings about duplicate paths

When generating a VM image from an installworld mtree manifest, makefs
spits out several thousand warnings about duplicate paths in the
manifest.  These are harmless and have been around for a long time (see
the phabricator revision for some more details), so let's at least have
a way to make makefs quieter.

Reviewed by:	brooks, imp, emaste
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D43513

usr.sbin/makefs/makefs.8

@@ -33,7 +33,7 @@
 .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 .\" POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd January 2, 2024
+.Dd January 19, 2024
 
 .Dt MAKEFS 8
 .Os
@@ -110,6 +110,8 @@ suffix may be provided to indicate that
 indicates a percentage of the calculated image size.
 .It Fl D
 Treat duplicate paths in an mtree manifest as warnings not error.
+If this flag is specified more than once, warnings about duplicate paths
+are not printed at all.
 .It Fl d Ar debug-mask
 Enable various levels of debugging, depending upon which bits are
 set in

usr.sbin/makefs/makefs.c

@@ -164,7 +164,7 @@ main(int argc, char *argv[])
 			break;
 
 		case 'D':
-			dupsok = 1;
+			dupsok++;
 			break;
 
 		case 'd':

usr.sbin/makefs/mtree.c

@@ -894,11 +894,11 @@ read_mtree_spec1(FILE *fp, bool def, const char *name)
 
 		if (strcmp(name, node->name) == 0) {
 			if (def == true) {
-				if (!dupsok)
+				if (dupsok == 0)
 					mtree_error(
 					    "duplicate definition of %s",
 					    name);
-				else
+				else if (dupsok == 1)
 					mtree_warning(
 					    "duplicate definition of %s",
 					    name);