Changed default settings to bring a better gameplay.

Hi, this is a rebased and redone pull request of https://github.com/minetest/minetest/pull/1648/
These  are some changes to the default settings that improve gameplay, performance and connection time.
Maybe we can tweak those settings on our own bus most people can't.

In this pull request I'm using the same values on the minetest.conf.example and defaultsettings, they were different.

* **always_fly_fast** is now set to false. The default behaviour when you are walking is to walk at normal speed and when you feel like going fast, just hold the 'e' key down. I find logical to keep the same behaviour when flying, especially when building large and tall structures, like castles, towers and skyscrapers. You wouldn't want to always run fast, so why would you want to fly always fast?

* Added a better description for dynamic range view on how to use it.

* **viewing_range_nodes_max** is now 30, the original was 35. This will help the vast majority of players who plays on old or slow computers. From experience on my server I learned that is not uncommon at all people playing on netbooks or laptops that have GPUs like VIA and S3.

* **viewing_range_nodes_min** is now 120, the original was 160. From experience of playing Minetest for years now, I learned that the dynamic adjustable will tend to push the limits too much and a too high view range is not worth it. This was confirmed by @paramat.

* **client_unload_unused_data_timeout** reduced from 600 to 300, This will help people with 2 GB of ram and even some people with 4 GB of ram. I already got my client killed by the operating system because it was using too much memory, that was solving reducing the unload time out. It will also help most people playing on slow and old devices.

* **fall_bobbing_amount** now set to 0.6. This somehow gives a nice feedback, feeling when you fall. I expect to have the action of gravity and weight when I jump from a tall place and fall.

* **curl_timeout** reduced from 5 seconds to 4 seconds. If the server didn't respond in 4 seconds it's unlikely that it will in 5. Besides that, one second does feel a lot of time when you are already waiting for your client to connect to a server.