libcommon|GameSession: Removed use of busy mode for saving the game, cleanup

Using busy mode for saving the game is somewhat pointless given that
it usually takes longer to perform the busy mode setup than it does
to serialized the game state to a new .save package.
libcommon: Cleanup

G_ApplyCurrentSessionMetadata() now defaults the SessionMetadata
user description to "(Unsaved)", for GameSession's convenience.
libcommon|GameSession: Always update rather than replace internal .save

For some reason FS2 is complaining that it cannot replace the Info
when performing an update even though the .save package is apparently
writable.
Fixed|libcommon|GameSession: Replacing internal .save Info

Following File::reinterpret() one must manually (re)populate()

IssueID #1791