Bug#963941: steam: Steam fails to start

[Simon McVittie]

Control: severity 963525 grave
Control: tag 963525 + upstream bullseye sid
Control: found 963525 1.0.0.33-1
Control: forcemerge 963525 963941

The reporter of #963941 confirmed in private email that libmount1 is at
version 2.35.2-6, so I'm merging it with the existing bug and raising
its severity in an attempt to avoid more duplicate reports.

I'm marking this as "found" in the oldest version of the steam package on
record, to indicate that it affects all versions equally (not a regression
in the steam package), and tagging it as "bullseye sid" to indicate that
it is not a practical problem in Debian 10 'buster' and older (because
they don't have the libmount1 that triggers it).

This isn't an ideal representation, but the Debian bug tracking system
is designed for tracking bugs that we might be able to fix, and we
are both legally and technically unable to fix this one without Valve
taking action (which would have to be in the form of a Steam update,
and would not necessarily involve an update to our package).

    smcv