Provide a mount option to drvfs to prevent dropping of executable permission via fmask on exe files

By default if a Windows file lacks WSL metadata, its NTFS permissions will be translated to unix permissions^1.
This effectively means all files will be marked as executable which is problematic and ugly (#936).

One can try to workaround by setting fmask=117 on mount point. This will drop executable bit from everything but allow user to put them back using chmod. However such thing will be impossible on system files such as explorer.exe due to lack of permissions.

I propose adding new mount flag, perhaps named nofmaskexe, to disable fmask on files matching *.exe.

This will enable partially emulating the behavior of msys/cygwin which is IMHO what most users expect.

Cygwin-like tools also detect executables by presence of shebang ("#!"), in addition to extension matching but this might be unnecessary here - it's unlikely to have unix style script in system directory.

I would like to see it in both WSL1 and WSL2.

Start by tracing how drvfs mount options and fmask are handled in both WSL1 and WSL2. Determine where executable-file permission translation occurs and how a nofmaskexe option would be represented; done means the option prevents fmask from dropping executable permission on .exe files in both environments.