Mainmenu: Still support favorites if send_pre_v25_init is disabled

Mainmenu: Still support favorites if send_pre_v25_init is disabled

@SmallJoker has noted a bug that servers from the (local) main menu
favorites list can't be opened.

This commit fixes the bug by disabling any main menu based protocol
checks for servers from the favorite list.

Also, it fixes a second bug that happens when a server from the
public serverlist doesn't send its supported protocol versions,
most likely because its running a minetest older than commit [1].
Then we have shown an error msg that the server has enforced
one specific protocol version. This was most likely not the case.

Of course, we can't do anything better than do an assumption on
the protocol versions if they are not known. That assumption
should however be closest to the most often occuring case as
possible.

Also, some little cleanups.

[1]: 5a0ed780f56a5225b3d7c5f64099586e390e5f39 "Server: announce MIN/MAX protocol version supported to serverlist. Client: check serverlist"