Fix umask default for DrvFs automount

[jjvliu]

Fixes #1628 and fixes #1767 so that the docs are consistent with the real behavior of WSL2. From testing, WSL has a default umask of 000, not 022.

Testing done

1. Create a new file: touch /mnt/c/temp/testfile.
2. Edit /etc/wsl.conf so that the [automount] section does not contain the key-value pair options = ... and save the file. This ensures the default umask is applied.
3. Run wsl.exe --shutdown.
4. Wait 8 seconds.
5. Open WSL and check the mode of the test file:

$ ls -lh /mnt/c/temp/testfile
-rwxrwxrwx 1 myusername myusername 0 Dec  0 09:33 /mnt/c/temp/testfile

Note that all the permission bits are set, meaning the default umask is 000: anyone can read, write, or execute files.

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[jjvliu]

Hi team, I updated the PR to resolve merge conflicts.

I'd also like to clarify what this is about. From testing, WSL has a default umask of 000, not 022, as mentioned in #1628:

In the table, default umask is said to be 000. In the text afterwards, it's said to be 022. From my experience, almost all files and directories from DrvFS are 777, so it seems the former is correct.

This also fixes a related inconsistency pointed out in #1767 (but by changing the following paragraph, not the table):

The default shown in the table above this was 000. At least one of these must be incorrect.

I added info about testing done to the original post.

Thanks!
Jimmy