BUG/MINOR: vars: fix set-var/unset-var exclusivity in the keyword parser

BUG/MINOR: vars: fix set-var/unset-var exclusivity in the keyword parser

The parser checks first for "set-var" then "unset-var" from the updated
offset instead of testing it only when the other one fails, so it
validates this rule as "unset-var":

    http-request set-varunset-var(proc.a)

This should be backported everywhere relevant, though it's mostly harmless
as it's unlikely that some users are purposely writing this in their conf!