If htpasswd was modified, it will be automatically reloaded. This check
happens at most once per second and only on incoming requests.
Note that this removes the public `NewHtpasswd()` function.
The Linux kernel page cache ALWAYS knows better. Fighting it brings
only worse performance. Usage of fadvise() is wrong 9 out of 10 times.
Removing the whole fs package brings a nice 100% speedup when running
costly prune command. And that is measured on localhost, the improvement
could be much bigger when using network with higher latency.
