MINOR: vars/cli: add a "get var" CLI command to retrieve global variables

MINOR: vars/cli: add a "get var" CLI command to retrieve global variables

Process-wide variables can now be displayed from the CLI using "get var"
followed by the variable name. They must all start with "proc." otherwise
they will not be found. The output is very similar to the one of the
debug converter, with a type and value being reported for the embedded
sample.

This command is limited to clients with the level "operator" or higher,
since it can possibly expose traffic-related data.