add "nosave" cvar token

[m8f]

It makes a cvar value not be saved in a save file.

Use case: saving global gameplay statistics. If it's saved into a savefile, loading this savefile will overwrite the updated statistics.

PR: https://github.com/coelckers/gzdoom/pull/1009

[m8f]

... And reverting this.

1. It turns out that it wasn't even needed. Saving server cvars in the save file can be easily overwritten with the value from ini.
2. nosave cvars are not saved not only to savefile, but to INI too. So such cvars could not be used for the intended purpose in the first place.

Sorry.

PR: https://github.com/coelckers/gzdoom/pull/1010

[Graf Zahl]

Too late for reverting it. We can either remove setting the flag or just keep it as a pointless feature.

[m8f]

Closer examination showed that this feature is actually needed.

Here is the working implementation.

PR: https://github.com/coelckers/gzdoom/pull/1011