Fix the loading screen for a ROM appearing for a split second after the splash screen.

Fix uninitialised memory access in Check_LoadArg.
Allow all audio buffers to get full again before declaring underflow.
Reinstate frame skipping options 0 and 1, but keep the new meaning (>= 2 equivalent skip level) for automatic frame skipping. Remove the automatic CPU frequency option, which was making audio emit 0.25 second of silence every so often.
Quit saving the files so often for nothing; that uses up erase cycles on storage cards! Instead, save them once when the menu that contains the settings that the user changed is exited, IF the settings' values changed, OR once when updating the list of most-recently played games.

Automatically save the SRAM in most cases, including game changes.

This commit also makes it unnecessary to save the game config of the previous game when loading another, makes it load certain settings correctly, and MAY make it avoid creating a file for a game's settings if the user never changes them from the defaults.