fix linux-centric segfaults, new check and install script

[rodan]

ChangeLog:

• fixed segfaults involving read-only 'CD_Files/*' opened read-write without any reason by myopen()
• fixed segfaults based on CreateOrOpenFile() trying to close a file that is NULL
• fixed segfaults based on an invalid language - the root cause is trying to open the default fallback language LANGUAGE/D*.TXT when the filesystem only has language/d2.txt (all lowercase). user needs to remove GAME/NETHERW/CONFIG.DAT and run tools/check_install.sh -d "data dir", see below.
• fixed non-existant music in linux based on wrong case for the SOUNDS directory

info to be added to the README:

in order to install the original game's data please run the following script:

bash tools/check_install.sh -s "directory where GOG installed MC2" -d "destination directory"

in order to check your current install, run:

bash tools/check_install.sh -d "destination directory"

where 'destination directory' is probably 'build/Debug/inst/bin/' if your current pwd is the repository root dir.

this 'tools/check_install.sh' script must supersede all prior linux install guides/commands/scripts.

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
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0.0% Coverage
0.0% Duplication

[rodan]

that extract-GOG-CD.sh is a minefield that the linux user does not need to walk thru, so I cleaned up the linux install steps.