MINOR: vars: add a "set-var-fmt" directive to the global section

MINOR: vars: add a "set-var-fmt" directive to the global section

Just like the set-var-fmt action for tcp/http rules, the set-var-fmt
directive in global sections allows to pre-set process-wide variables
using a format string instead of a sample expression. This is often
more convenient when it is required to concatenate multiple fields,
or when emitting just one word.