use mmap for checking rcfile

In response to #12:

- Cleaned up the changes and treat the `mmap()` checks only as a signal to activate the original cache update code.
- Only use when `DYNAMC_FAKETIMERC` flag is set. If you think this is not necessary, even better =)
- For perf reasons, it only checks `/etc/faketimerc` and not user `.faketimerc`s. Hopefully having `FAKETIMERC` in the compile flag name helps a bit with this clarification?
- I would much prefer this get merged rather than #13 cause this requries far fewer external changes