o/snapstate: move the reboot part of Backend.LinkSnap to a new method

[alfonsosanchezbeato]

Backend.LinkSnap prepares the system for a reboot, move that bit to a different backend method so we can use it from tasks different from link-snap, as we now plan to do reboots later that said task in some cases.

[alfonsosanchezbeato]

This is a preparatory PR to make the reboot later when updating a kernel with kernel-modules components.

[codecov]

Codecov Report

Attention: Patch coverage is 57.50000% with 17 lines in your changes missing coverage. Please review.

Project coverage is 78.94%. Comparing base (2e47491) to head (afde847).
Report is 28 commits behind head on master.

Files with missing lines	Patch %	Lines
overlord/snapstate/handlers.go	57.14%	6 Missing and 3 partials ⚠️
overlord/snapstate/backend/link.go	57.89%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #14641      +/-   ##
==========================================
+ Coverage   78.89%   78.94%   +0.05%     
==========================================
  Files        1083     1084       +1     
  Lines      146377   146648     +271     
==========================================
+ Hits       115479   115768     +289     
+ Misses      23695    23676      -19     
- Partials     7203     7204       +1     

Flag	Coverage Δ
unittests	78.94% <57.50%> (+0.05%)	⬆️

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