Move tracking of user-managed checkpoints into replayer.cc.
Handle EPERM response to SIOCETHTOOL and SIOCGIWRATE ioctls.
Move the pull request checklist to the canonical github file for it

Having the instructions in this file will make the github pull
request UI show a notice when users submit their PR:

https://github.com/blog/1184-contributing-guidelines
Rename suppress_performance_warnings flag to suppress_warnings.
Every time we record or replay, try to detect the presence of the "CPUID causes rbc undercount" bug.
Automatically apply rcb slack when we detect the "CPUDID rcb undercount bug".
Make the test failure message clearer.