- improve performance by optimizing common sequences of system calls.
  For example, gettimeofday() can probably safely be made to evaluate lazily.

- for better efficiency, try to find instances of RDTSC in glibc and rewrite
  them. That would avoid the signal handling, when it is not strictly
  necessary.

- try to find places where we call any of the unrestricted system calls and
  avoid instrumenting the code.

- audit error code handling in trusted_thread.cc.

- get rid of preloader and of ELF helper process.

- clean up sandbox_impl.h, maybe create extra helper classes?

- address (or remove) remaining "TODO(markus):" statements.
