Many DOS games open all their files in write-mode regardless if the
game actually intends to write to them or not.
This can result the files' metadata being updated (lossing the
original date-stamp) as well as putting the file at-risk of
corruption if the game crashes with the file open for writing.
Under the existing DOSBox implementation, if a user attempts to
write-protect their data files then DOSBox will quit with an error in
the above scenario.
This commit allows the use of write-protected files (similar to
running a game from CDROM) and indicates which files are being
actively protected against modification.
Typically the only files that need to be left writable are those that
the game actually changes, such as: save-game files, highscore
files, in-game settings, and so on. If these are also protected,
then the game will quit/crash immediately after the protected message
is printed, and thus indicate which file require write-allowance.
Judging by the usage, this header belongs in there instead of being
limited to dos module only. This change makes it easier to reuse code
for new features related to drives mounting/unmounting.
