Fix some simple yet devastating bugs.

1. Remove the check for emptiness of (cell & globals) as function arguments can be in both at the same time.
2. Use precached `argspec._flags` in addition to `OPTIMIZED` and `NEWLOCALS`. Fixes varargs and varkwargs.
3. Fix a typo in `argspec._flags` calculation.
4. Allow already closed variables to be used in a second local function.
5. Always add function arguments to `locals`, regardless of whether they were moved to `cell`.
6. Fix imported name calculation in `import`. `import '/sys/stdin'` should no longer store `stdin` as `sys`.