nfsmount: call mount_nfs directly on OpenBSD so -T is accepted

The previous OpenBSD path built the right options ("-o port=N -T") but
still handed them to mount(8). On OpenBSD "-T" is a mount_nfs(8) flag,
not a mount(8) one, so mount rejected it with "mount: unknown option --
T" and the mount never ran. Call mount_nfs(8) directly on OpenBSD; the
options are already in its native syntax.

Verified on a real OpenBSD 7.9 arm64 VM: with this change the command
becomes "mount_nfs -o port=N -T localhost:/ <mnt>" and mount_nfs accepts
the flags. The mount then fails later with "Stale NFS file handle"
because the OpenBSD client and rclone's in-process NFS server disagree
on the root filehandle - that is a separate issue in the NFS server,
not in the mount options this PR is about.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

cmd/nfsmount/nfsmount.go

@@ -54,16 +54,20 @@ func mount(VFS *vfs.VFS, mountpoint string, opt *mountlib.Options) (asyncerrors
 		return
 	}
 
-	// Options
+	// Options and mount binary
 	//
 	// OpenBSD's mount_nfs(8) doesn't understand "-o mountport=" or "-o
 	// tcp" - the mountd port isn't settable that way and TCP is
-	// requested with the "-T" flag instead, so build a different set of
-	// options there. FreeBSD's mount_nfs(8) accepts the same "-o
-	// port=", "-o mountport=" and "-o tcp" options as Linux so it uses
-	// the common case.
+	// requested with the "-T" flag instead. "-T" is a mount_nfs(8) flag,
+	// not a generic mount(8) one, so on OpenBSD we call mount_nfs(8)
+	// directly; running it through mount(8) makes mount reject "-T" with
+	// "unknown option". FreeBSD's mount_nfs(8) accepts the same "-o
+	// port=", "-o mountport=" and "-o tcp" options as Linux, and mount(8)
+	// there forwards them, so it stays on the common path.
+	mountBin := "mount"
 	var options []string
 	if runtime.GOOS == "openbsd" {
+		mountBin = "mount_nfs"
 		options = []string{
 			"-o", fmt.Sprintf("port=%s", port),
 			"-T",
@@ -84,7 +88,7 @@ func mount(VFS *vfs.VFS, mountpoint string, opt *mountlib.Options) (asyncerrors
 	if sudo {
 		cmd = append(cmd, "sudo")
 	}
-	cmd = append(cmd, "mount")
+	cmd = append(cmd, mountBin)
 	cmd = append(cmd, options...)
 	cmd = append(cmd, "localhost:"+mountPath, mountpoint)
 	fs.Debugf(nil, "Running mount command: %q", cmd)