interfaces/wayland: add wayland interface

[jdstrand]

This implicit classic interface is very simple-- add an AppArmor rule for owner /run/user/*/wayland-[0-9]* rw, and have the base declaration allow anyone to autoconnect to it.

The previous investigative PR tried to undo snapd's setting of XDG_RUNTIME_DIR, but this is no longer required. Because we aren't changing XDG_RUNTIME_DIR, there is no risk in breaking existing clients, but clients won't be able to find the wayland server socket without a little help. The help given is similar to what the desktop part is doing for dconf and thus acceptable for now, IMO.

Future iterations of the interface might build upon the portals work and be part of the per-user mounts that is being discussed. This PR does not introduce complications for that work.

[zyga]

LGTM

[zyga]

Thank you :-)

[zyga]

@jdstrand the test failure looks relevant:

----------------------------------------------------------------------
FAIL: basedeclaration_test.go:127: baseDeclSuite.TestAutoConnection
basedeclaration_test.go:174:
    c.Check(err, NotNil, comm)
... value = nil
... wayland

Which reminds me, I'll move the tests for the base declaration for interfaces to the interfaces/builtin package where people often go test while they work. This will make it less likely to miss changes to base declaration tests.

[jdstrand]

Thanks for this.

[codecov-io]

Codecov Report

Merging #3690 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3690      +/-   ##
==========================================
+ Coverage   75.29%   75.33%   +0.04%     
==========================================
  Files         389      390       +1     
  Lines       33683    33675       -8     
==========================================
+ Hits        25361    25370       +9     
+ Misses       6503     6487      -16     
+ Partials     1819     1818       -1

Impacted Files	Coverage Δ
interfaces/builtin/wayland.go	100% <100%> (ø)
cmd/snap-seccomp/main.go	49.79% <0%> (ø)	⬆️
overlord/snapstate/snapstate.go	80.46% <0%> (+0.26%)	⬆️
overlord/ifacestate/helpers.go	63% <0%> (+0.66%)	⬆️
interfaces/builtin/physical_memory_observe.go	100% <0%> (+42.85%)	⬆️

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