many: run component install hooks

[andrewphelpsj]

This PR is the final piece that allows us to run install hooks for components. This change contains the snap-confine changes. The majority of changes in snap-confine deal with parsing security tags that are generated for component hooks.

An example of a security tag from a component hook would be: snap.name+comp.hook.install
And one with an instance key: snap.name_instance+comp.hook.install

Something important to note is how these are encoded as udev tags. Currently, when converting a security tag to a udev tag, we replace all . characters in the tag with _ characters because systemd limits udev tags to having only alphanumeric characters, with the addition of the characters - and _. Since security tags can now contain +
characters, those will be encoded as two consecutive _ characters.

For example:

"snap.name+comp.hook.install" -> "snap_name__comp_hook_install"
"snap.name_instance+comp.hook.install" -> "snap_name_instance__comp_hook_install"

This allows the conversion to maintain its reversibility.

[bboozzoo]

shellcheck appears to be unhappy:

In tests/main/component-hooks/comp-with-install-hook/meta/hooks/install line 5:
printf "GET /ip HTTP/1.0\r\n\r\n" | nc httpbin.org 80 2>&1 > "${SNAP_COMMON}/install-logs" 
                                                      ^--^ SC2069 (warning): To redirect stdout+stderr, 2>&1 must be last (or use '{ cmd > file; } 2>&1' to clarify).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.99411% with 168 lines in your changes are missing coverage. Please review.

Project coverage is 78.73%. Comparing base (1bad859) to head (9678627).
Report is 3 commits behind head on master.

Files	Patch %	Lines
interfaces/ifacetest/backendtest.go	0.00%	64 Missing ⚠️
interfaces/ifacetest/app_set.go	28.57%	48 Missing and 2 partials ⚠️
interfaces/repo.go	61.70%	12 Missing and 6 partials ⚠️
cmd/snap/cmd_run.go	85.41%	9 Missing and 5 partials ⚠️
interfaces/connection.go	33.33%	8 Missing ⚠️
interfaces/snap_app_set.go	93.24%	4 Missing and 1 partial ⚠️
cmd/snap-exec/main.go	90.00%	2 Missing and 1 partial ⚠️
interfaces/kmod/backend.go	71.42%	0 Missing and 2 partials ⚠️
daemon/api_sideload_n_try.go	0.00%	1 Missing ⚠️
interfaces/builtin/modem_manager.go	50.00%	1 Missing ⚠️
... and 2 more

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Additional details and impacted files

@@            Coverage Diff             @@
##           master   #13775      +/-   ##
==========================================
- Coverage   78.83%   78.73%   -0.11%     
==========================================
  Files        1043     1044       +1     
  Lines      134470   135082     +612     
==========================================
+ Hits       106012   106357     +345     
- Misses      21847    22076     +229     
- Partials     6611     6649      +38     

Flag	Coverage Δ
unittests	78.73% <66.99%> (-0.11%)	⬇️

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