fix(fs, model): improve symlink resilience in file shortcut (#10739)

Ensure file was a file before the shortcut as well as after... (This was
implied when talking to a correct implementation, but not enforced.)

Make our file opening operations safe by default by ensuring the last
path component is not a symlink.

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/fs/basicfs_unix.go

@@ -14,8 +14,11 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
+	"syscall"
 )
 
+const alwaysOpenFlags = syscall.O_NOFOLLOW // never open symlinks as the final path component
+
 func (f *BasicFilesystem) CreateSymlink(target, name string) error {
 	name, err := f.rooted(name)
 	if err != nil {