Need way to (at least) declare symlink and link always return ENOSYS

[dsheets]

When file systems are under development, it's common to want to test them without even pretending to implement symlink and link. Because the current test suite does not contain minimized test cases, many tests use one or the other syscall in their environment set-up but are not dependent on the effects. This causes lots of unrelated tests to fail when these VFS calls aren't implemented.