Add a "Hint Mode ref (+ | -)*" hint for setting a global mode
of resulution for goals whose head is "ref". + means the argument
is an input and shouldn't contain an evar, otherwise resolution
fails. This generalizes the Typeclasses Strict Resolution option
which prevents resolution to fire on underconstrained typeclass
constraints, now the criterion can be applied to specific parameters.
Also cleanup auto/eauto code, uncovering a potential backwards
compatibility issue: in cases the goal contains existentials, we
never use the discrimination net in auto/eauto. We should try to
set this on once the contribs are stabilized (the stdlib goes through
when the dnet is used in these cases).