Version 6:
 - don't use fputc(), fputs(), fprintf() or fflush() on stderr: use write() on file descriptor 2 (should be stderr)
 - write tests: add sigsegv(), sigfpe() and fatal_error() functions to the _testcapi module

I was too lazy to reimplement functions to convert an integer to a string in bases 10 and 16, so I used snprintf() on a small buffer allocated on the stack.