Log Message:
Updated afl to 2.21b.

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Version 2.21b:
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  - Added some crash reporting notes for Solaris in docs/INSTALL, as
    investigated by Martin Carpenter.

  - Fixed a minor UI mix-up with havoc strategy stats.

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Version 2.20b:
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  - Revamped the handling of variable paths, replacing path count with a
    "stability" score to give users a much better signal. Based on the
    feedback from Vegard Nossum.

  - Made a stability improvement to the syncing behavior with resuming
    fuzzers. Based on the feedback from Vegard.

  - Changed the UI to include current input bitmap density along with
    total density. Ditto.

  - Added experimental support for parallelizing -M.