General cleanup of command line parsing to allow "busybox" to work as a prefix.

General cleanup of command line parsing to allow "busybox" to work as a prefix.
(I.E. any argv[0] that starts with "busybox" winds up in busybox_main().)

Added testing/busybox.tests which tests the following permutations:

./busybox
./busybox-suffix
./busybox cat
./busybox-suffix cat
./busybox --help
./busybox-suffix --help
./busybox --help cat
./busybox-suffix --help cat
./busybox --help unknown
./busybox-suffix --help unknown
./unknown

Also repair the test suite so ./runtest calls the ".tests" scripts properly.

Note: you can now go "busybox busybox busbox ls -l" and it'll take it.  The
new code is pretty generic.  I can block that if anybody can come up with a
good reason to...