Shouldn't allow creating a Steam library in locations that won't work

Your system information

Steam client version (build number or date): any
Distribution (e.g. Ubuntu): any
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Please describe your issue in as much detail as possible:

There are several reserved filesystem paths that can't be used by the Steam Linux Runtime container runtime framework. Increasingly many games being run on Linux are either Windows games via Proton or native Linux games via Steam Linux Runtime 3.0 (sniper), which won't work if Proton, Steam Linux Runtime or the game itself are installed into a library in one of these reserved locations.

The container runtime doesn't have control over its own install location, and doesn't have any user interface other than logging, so it isn't a good place to handle this error. The Steam client should not allow these reserved locations to be used as locations for Steam libraries, with a user-facing error message (or at least a warning).

The affected paths are mainly /etc and /usr/local, but for completeness, none of these should be allowed:

/app (non-standardized path, reserved because we recycle code from Flatpak)
/bin
/boot
/dev
/etc
/lib* (including /lib32, /lib64, /libx32)
/overrides (non-standardized path, used internally by the Steam Linux Runtime)
/proc
/root
/sbin
/sys
/usr (including /usr/local)
/var

Most of /run and /var/run also won't work, but there are exceptions to those: /run/flatpak/doc, /run/media and /run/user/*/doc should be OK.

In particular, I expect Steam libraries below /run/media/ to be common, especially on Steam Deck, so Steam definitely does need to allow those.

Steps for reproducing this issue:

For example see https://github.com/ValveSoftware/steam-runtime/issues/621, https://github.com/ValveSoftware/steam-runtime/issues/288.

ValveSoftware / steam-for-linux