login: T4975: Fixed broken CLI commands

[zdc]

Change Summary

Fixed broken CLI commands

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://phabricator.vyos.net/T4975

Component(s) name

login, CLI

Proposed changes

User profile files are not saved to disk after configuration is fully applied. Because of this, after a fast system reset, profile files can be empty, and CLI is broken.

This fix adds a sync() call after the user's configuration, which should protect from data loss and fix the problem with profiles.

How to test

1. Boot VyOS for the first time (after installation or from VM image).
2. After full boot, reset the VM.
3. Wait for the boot and log in.
4. Check the CLI.
5. VyOS-specific commands will not work.

vyos@vyos:~$ show 
showconsolefont  showkey          
vyos@vyos:~$ show interfaces

  Invalid command: [show]

vyos@vyos:~$

After the fix, everything should be OK.

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