vbash: beautify tab completion output/line breaks

[c-po]

Change Summary

Commit a6f82bb ("T1748: vbash: beautify tab completion output/line breaks")
added a method to split the help string and insert newlines and leading tabs
in a deterministic way.

This commit cleans up the legacy implementations where leading whitespaces got
counted and added by humans in a try/error method.

Change from

cpo@LR2.wue3# set service dhcp-server
Possible completions:
   disable      Temporary disable
   dynamic-dns-update
                Dynamically update Domain Name System (RFC4702)
 > failover     DHCP failover configuration
+  global-parameters
                Additional global parameters for DHCP server. You must
                use the syntax of dhcpd.conf in this text-field. Using this
                without proper knowledge may result in a crashed DHCP server.
                Check system log to look for errors.
   host-decl-name
                Use host declaration name for forward DNS name
   hostfile-update
                Updating /etc/hosts file (per client lease)
+  listen-address
                Local IPv4 addresses for service to listen on
+> shared-network-name
                Name of DHCP shared network

to

cpo@LR1.wue3# set service dhcp-server
Possible completions:
   disable              Disable instance
   dynamic-dns-update   Dynamically update Domain Name System (RFC4702)
 > failover             DHCP failover configuration
+  global-parameters    Additional global parameters for DHCP server. You must use the
                        syntax of dhcpd.conf in this text-field. Using this without
                        proper knowledge may result in a crashed DHCP server. Check
                        system log to look for errors.
   host-decl-name       Use host declaration name for forward DNS name
   hostfile-update      Updating /etc/hosts file (per client lease)
+  listen-address       Local IPv4 addresses to listen on
+> shared-network-name  Name of DHCP shared network

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/T1748
• T1748: vbash: beautify tab completion output/line breaks vyatta-cfg#48

Component(s) name

bash

Proposed changes

How to test

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