nfsmount: fix mount_nfs options incompatible with OpenBSD - fixes #8578

nfsmount built its mount options for GNU/Linux syntax unconditionally:
"-o port=N", "-o mountport=N" and "-o tcp". OpenBSD's mount_nfs(8)
rejects "-o mountport" outright ("option not supported", per the
reporter's log) and has no "tcp" suboption either, since it selects
TCP with the separate "-T" flag instead of an -o suboption.

Add a runtime.GOOS == "openbsd" branch that builds the option list
OpenBSD's mount_nfs actually accepts: "-o port=N" plus "-T" for TCP,
with no mountport option since OpenBSD's mountd is located via
portmap rather than a fixed, settable port. This follows the same
GOOS-branching pattern already used in this file's unmount function
(darwin) and in cmd/cmount/mount.go for openbsd/freebsd differences.

FreeBSD's mount_nfs(8) documents "port=", "mountport=" and "tcp" as
-o suboptions identical to Linux, so the existing option set is left
unchanged for freebsd and all other platforms.

Verified by cross-compiling (go build and go vet) for GOOS=openbsd,
freebsd, linux and darwin, all of which succeed. Actually mounting
via mount_nfs on OpenBSD needs a BSD machine to confirm at runtime,
which wasn't available here.

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

cmd/nfsmount/nfsmount.go

@@ -55,10 +55,25 @@ func mount(VFS *vfs.VFS, mountpoint string, opt *mountlib.Options) (asyncerrors
 	}
 
 	// Options
-	options := []string{
-		"-o", fmt.Sprintf("port=%s", port),
-		"-o", fmt.Sprintf("mountport=%s", port),
-		"-o", "tcp",
+	//
+	// 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.
+	var options []string
+	if runtime.GOOS == "openbsd" {
+		options = []string{
+			"-o", fmt.Sprintf("port=%s", port),
+			"-T",
+		}
+	} else {
+		options = []string{
+			"-o", fmt.Sprintf("port=%s", port),
+			"-o", fmt.Sprintf("mountport=%s", port),
+			"-o", "tcp",
+		}
 	}
 	for _, option := range opt.ExtraOptions {
 		options = append(options, "-o", option)