Split quick_exit into ok and not_ok versions

This allows a test to quit and determine whether the quit should
count as a 'pass' or a 'fail'.
For example:
    chflags does not exist in Linux. For chflags tests, quick_exit_ok
        should be used, because this does not show a deficiency in the
        filesystem, but rather the kernel.
    ACLs are unimplemented on some filesystems. For these tests,
        quick_exit_not_ok ought to be used because the lack of ACLs
        represent a lack of functionality at the filesystem level.

Also, a string may be passed to either of these functions which will
be echoed (and may be seen with 'prove -v'). This can be used to
give an explanation for the quick_exit_* invocation.