interfaces/many: miscellaneous updates based on feedback from the field (2.29)

[zyga]

This is a backport of #4097 for 2.29:

interfaces/desktop,unity7: allow read on system and session xdg user-dirs files
interfaces/browser-support: allow /run/udev access for cuse and vhost-vsock
interfaces: allow directory read on /home/ and / by default
interfaces/removable-media: allowing read on /run/ and /run/media/
interfaces/browser-support: also allowing reading /etc/chromium/{,**}
interfaces/browser-support: allow /proc/pid/mount{s,info} to avoid confusion
interfaces/home: allow reading files and dirs that start with 'snap[^/]'
interfaces/bluez: don't mediate 'path' to/from unconfined

[codecov-io]

Codecov Report

Merging #4101 into release/2.29 will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           release/2.29    #4101   +/-   ##
=============================================
  Coverage         75.78%   75.78%           
=============================================
  Files               433      433           
  Lines             37229    37229           
=============================================
  Hits              28215    28215           
  Misses             7042     7042           
  Partials           1972     1972

Impacted Files	Coverage Δ
interfaces/builtin/removable_media.go	100% <ø> (ø)	⬆️
interfaces/builtin/browser_support.go	76.31% <ø> (ø)	⬆️
interfaces/builtin/hardware_observe.go	100% <ø> (ø)	⬆️
interfaces/builtin/home.go	100% <ø> (ø)	⬆️
interfaces/builtin/unity7.go	67.85% <ø> (ø)	⬆️
interfaces/builtin/bluez.go	100% <ø> (ø)	⬆️
interfaces/builtin/desktop.go	86.84% <ø> (ø)	⬆️

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