Savegame Tool: Support conversion of vanilla Doom and Heretic savegames

Converting savegames in the vanilla Doom V9 and Heretic V13 formats
is now supported provided that the game identity key is specified on
the command line (it is not possible to determine this unambiguously
from the original file contents).

As these vanilla savegame formats omit the majority of the game rules
it may be necessary to assume their values from the current session
configuration at runtime.

In the future, the Doomsday UI could allow setting these explicitly.

For recognition purposes, such translated savegames use new "magic"
identification bytes.

Todo: Update libcommon to recognize translated vanilla savegames.