utils.cfunc: Cleanup, more flexibility

- Split code using ctypes directly into a helper class
- Don't load “libc.so.6”, but use handle for main program instead (see
  comment in code)
- Clarify comment on errno with older ctypes versions
- Rename unittest since it can't be used for other functions (modifies
  process environment at runtime)
- Add boolean return value for “Mlockall”

These changes are leftovers from some experiments with ctypes.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>