Updated afl to 2.51b.

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Version 2.51b (2017-08-30):
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  - Made afl-tmin call setsid to prevent glibc traceback junk from showing
    up on the terminal in some distros. Suggested by Jakub Wilk.

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Version 2.50b (2017-08-19):
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  - Fixed an interesting timing corner case spotted by Jakub Wilk.

  - Addressed a libtokencap / pthreads incompatibility issue. Likewise, spotted
    by Jakub Wilk.

  - Added a mention of afl-kit and Pythia.

  - Added AFL_FAST_CAL.

  - In-place resume now preserves .synced. Suggested by Jakub Wilk.

(wiz)