fix(mount): add -posixDirNlink option for correct directory nlink

Add optional POSIX-compliant directory nlink counting
(nlink = 2 + number_of_subdirectories) behind the -posixDirNlink
flag. This requires listing cached directory entries on every stat,
which has a performance cost, so it's off by default.

When disabled (default), directories report nlink=2, which is the
POSIX-compliant baseline for empty directories and a common default
in distributed filesystems.

- Set nlink baseline to 2 for all directories (was 1)
- Add applyDirNlink() gated behind option.PosixDirNlink
- Add -posixDirNlink CLI flag

weed/command/mount_std.go

@@ -354,6 +354,7 @@ func RunMount(option *MountOptions, umask os.FileMode) bool {
 		DirIdleEvictSec:   *option.dirIdleEvictSec,
 		EnableDistributedLock: option.distributedLock != nil && *option.distributedLock,
 		WritebackCache:       option.writebackCache != nil && *option.writebackCache,
+		PosixDirNlink:        option.posixDirNlink != nil && *option.posixDirNlink,
 	})
 
 	// create mount root