Update to nightly
Add a 'busytest' target to run uutils under the busybox testsuite

It requires the BUSYBOX_SRC environment variable to be set,
and optionally passes arguments to the busybox test runner with
the RUNTEST_ARGS environment variable.

Example:

```
make busytest MULTICALL=1 BUSYBOX_SRC=~/dev/busybox RUNTEST_ARGS=tr
```
Output list of uutils functions in a way that busybox's 'runtest' understands
Improve busybox compatibility for command line processing