MEDIUM: log: rename logformat var to logformat tag

MEDIUM: log: rename logformat var to logformat tag

What we use to call logformat variable in the code is referred as
log-format tag in the documentation. Having both 'var' and 'tag' labels
referring to the same thing is really confusing. Let's make the code
comply with the documentation by replacing all logformat var/variable/VAR
occurences with either tag or TAG.

No functional change should be expected, the only visible side-effect from
user point of view is that "variable" was replaced by "tag" in some error
messages.