We now have an `#include <cxxabi.h>` header which defines all the APIs
Cosmopolitan's implemented so far. The `cosmocc` README.md file is now
greatly expanded with documentation.
This change refactors our POSIX signals emulation for Windows so that it
performs some additional safety checks before calling SetThreadContext()
which needs to be locked and must never ever interrupt Microsoft's code.
Kudos to the the Go developers for figuring out how to do this properly.