T5996: selectively escape and restore single backslashes in config (backport #3035)

[mergify]

Change Summary

Gory details are summarized in the task, but the short story is that the legacy and modern parser and config output have conflicting practices in handling unescaped backslashes --- the existing filter to allow parsing of legacy output requires a refinement, in order to maintain the occasional use of single backslashes in values.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T5996

Related PR(s)

Component(s) name

Proposed changes

How to test

vyos@vyos# set interfaces ethernet eth0 description 'fo\o'
[edit]
vyos@vyos# compare
[interfaces ethernet eth0]
+ description "fo\o"

[edit]
vyos@vyos# commit
[edit]
vyos@vyos# run show configuration commands |match desc
set interfaces ethernet eth0 description 'fo\o'
[edit]
vyos@vyos# set interfaces ethernet eth0 description 'fo\\o'
[edit]
vyos@vyos# compare
[interfaces ethernet eth0]
- description "fo\o"
+ description "fo\\o"

[edit]
vyos@vyos# commit
[edit]
vyos@vyos# run show configuration commands |match desc
set interfaces ethernet eth0 description 'fo\\o'

Smoketest result

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

---

This is an automatic backport of pull request #3035 done by [Mergify](https://mergify.com).