Get rid of signal() hack for control flow collection

[kenballus]

Right now, control flow is collected by sending signals to afl-showmap. This is not only a hack, but inaccurate because the signal to start tracing must be sent before the fuzzed input (i.e., the request).

A better way to do this would be to make a little LD_PRELOAD library that hooks accept() and makes it automatically signal the parent process. This should cut out a lot of the noise from the coverage.