feat: Add atomic distribution support

[mrvictory1]

This PR adds support for Linux systems where /usr is read only by utilizing /usr/local which is writable on Universal Blue based distributions. The PR has been tested on Bazzite. List of changes:

• cli/CMakeLists.txt: C++ standard has been upgraded from C++ 11 to C++ 17. This is necessary to use std::filesystem.
• cli/src/main.cc: new logic has been added to check if /usr/bin/steam is modifiable. If it is not, an attempt is made to create a copy at /usr/local/bin. If it is successful, patching is performed in /usr/local/bin/steam. If it fails, millennium exits.
• scripts/install.sh: Script checks if /usr/bin is writable, if not it /usr/local/bin is checked. If that also fails, script exits.
• scripts/posix/start.sh: .so file is searched in both /usr/local/bin and /usr/bin. Lack of file in either directory is harmless.
• src/sys/env.cc: If /usr/local/bin/millennium exists, /usr/ is changed to /usr/local (done 3 times)

Todo / Caveats:

• /usr/local is not writable on some distributions ie bootc based CentOS. I did not know this when starting working on this. ~/.local/bin can be used instead.
• check_patch_status() in millennium cli does not check for /usr/local.
• Bazzite uses a custom script /usr/bin/bazzite-steam to launch Steam on desktop. This needs to be changed to /usr/bin/local/steam or simply steam which is not prevented by read only /usr.

[shdwmtr]

We probably shouldn't use filesystem in the CLI. its a huge dependency and likely brings significant overhead onto the stack which would increase the binary size.

Pretty sure copy_file_range or sendfile would work just as well!

[timelord1102]

Cant seem to reproduce an install on Bazzite. Files are placed properly by install.sh, but start.sh does not actually apply millennium, instead opening steam with no millennium options.

[mrvictory1]

@timelord1102 exit steam fully then start steam from the terminal with /usr/local/bin/steam or just steam. See point

Bazzite uses a custom script /usr/bin/bazzite-steam to launch Steam on desktop. This needs to be changed to /usr/bin/local/steam or simply steam which is not prevented by read only /usr.