cmount, mount, mountlib: make --read-only reject modify operations

Normally mount/cmount use `-o ro` to get the kernel to mark the fs as
read only.  However this is ignored by WinFsp, so in addition if
`--read-only` is in effect then return EROFS ("Read only File System")
from all methods which attempt to modify something.

cmd/mountlib/fs.go

@@ -39,6 +39,7 @@ type FS struct {
 	root       *Dir
 	noSeek     bool // don't allow seeking if set
 	noChecksum bool // don't check checksums if set
+	readOnly   bool // if set FS is read only
 }
 
 // NewFS creates a new filing system and root directory
@@ -66,6 +67,13 @@ func (fsys *FS) NoChecksum() *FS {
 	return fsys
 }
 
+// ReadOnly sets the fs into read only mode, returning EROFS for any
+// write operations.
+func (fsys *FS) ReadOnly() *FS {
+	fsys.readOnly = true
+	return fsys
+}
+
 // Root returns the root node
 func (fsys *FS) Root() (*Dir, error) {
 	// fs.Debugf(fsys.f, "Root()")