Utilise `execversion` to one-time reset two cvars on upgrade to v1.4.

- `cv_masterserver` - Replaces a hack checking for the original SRB2 MS addres every time, and additionally catches those who tested an early version of the http MS.
- `cv_usegamma` - This was previously WAY too easy to accidentially change. This way, a user who has had this accidentially happen will automatically return to baseline.