Prevent players from being kicked due to "bad" names

The issue

• If you try to join a netgame with single-letter names or names that have a character between spaces/space followed by a single character (names such as Sonic 2, E, etc), and there is already someone in that server with the same name, this is what will happen:

Single letter name

Name with spaces within

The solution

• Changes were made in d_netcmd.c's EnsurePlayerNameIsGood function, in order to ensure that trailing spaces from names are properly removed and that players with single-letter names get assigned another name that prevents them from being kicked.

Single letter name

Name with spaces within