T5137: Refactoring show tech-support report

Split script to small functions for flexible output reports.
Improve header for commands.
Each funciton easily can be modified or extended.
Remove splitting command/output via percent symbol.
Remove old commands and directiories like /etc/rc.local, iptables,
brctl, etc.
Remove ethtool operation for subinterfaces.
Extend ethtool debug output.
Add correct nftables command.