use local scope for private function vars
note, this builds upon and thus bundles some commits from MRs #112, #115 and #117
all vars affected have been carefully checked to be quite certain that they are definitely local
where variable is assigned the return value of a function/command, the
local "declaration" is deliberately done on a separate line, since
local FOO is actually treated itself as a command rather than a
declaration; will thus always cause $? to be zero, and thus if done on
the same line as such an assignment can not only clobber $? but in doing
so unintentionally blocks failure of a command from triggering the
expected exit from having set -e.
also, from testing, i have found that when assigning "${@}" this must be
done on a separate line confusingly as otherwise an error occurs.
i've tried this out in a test run and encountered no issues beyong the "${@}" one just mention which i've addressed.