Implement var_szuser1, var_szuser2, var_szuser3, var_szuser4

[JUNESYNGOTOFLEX]

useful to create custom weapons

[StevenKal]

The public structure "entvars", has no such fields, despite we can consider the "noise*" ones like "user", but there are sometimes used by the games, and in overal, all of those "*user" fields should only be reserved to engine & games, not for plugins's storage. For plugins, the best is to use some kind of "Entity Custom Data" (which exists).