Extend the allowed protocol versions of strict servers

The current strict protocol check is not optimal. 0.4.10 clients can not connect to a strict 0.4.10-dev server. [untested]

Example: https://forum.minetest.net/viewtopic.php?p=157728#p157728
This pull allows clients to connect if their protocol version is in the range of +/- 1

I understand the meaning of "strict", but adding an additional setting for this is not required.
~~An other way would be to use `strict_protocol_version_checking` as "allowed range".~~
Added in a 2nd commit. [untested]
This makes it impossible to get the old strict behavior.  Instead, you could have negative values disable it, or not add it as a default setting and have non-existence indicate that it should be disabled.