Now the description includes the variable scope, `set [-e] -[Ugl]`
completions only provide variables matching that scope, and completions
that shouldn't be modified are hidden from the user. Completions that
are often modified but rarely unset (`fish_*` variables) are omitted
from `set -e` completions.
A new helper function `__fish_seen_argument` has been added that makes
it easy to only provied completions for a specific flag.
