Add DofMap::(un)stash_dof_constraints()

Adjoint-based sensitivity calculations can require us to calculate
residuals without applying constraints, and this seems to be the least
modularity-defiling way to let us turn them off and back on quickly.
adjoint_qoi_sensitivity bugfix for adjoint BCs

With heterogeneous adjoint BCs we can't constrain the residual vector
and still expect to accurately calculate adjoint-weighted residuals
with a dot product.
Fix lift_func errors in adjoint qoi sensitivity

Choosing the right BCs and not making a sign error might be helpful...
Use API properly

enforce_constraints_exactly took a pointer to allow for a default
NULL; but enforce_adjoint_constraints exactly takes a reference...
has_heterogenous_adjoint_constraint changes

New method has_heterogenous_adjoint_constraints tests for any such
constraints.

Old method has_heterogenous_adjoint_constraint new returns the
constraint rhs Number rather than bool; this should be semantically
backward-compatible with the old system unless someone was manually
setting a zero constraint and expecting that to count as heterogenous.
Integrate regularized flux QoIs in FEMSystem
FEMSystem integration of heterogeneous qoi_deriv

This is consistent with the new assemble_qoi behavior
Selectively apply liftfunc and constraints

We need the liftfunc terms to be left out of the RHS for adjoint
solves; we need QoI derivative vectors to be left unconstrained when
using them for dot products in sensitivity calculations.
Fix a bug Vikram caught in FEMSystem qoi derivs
Merge pull request #345 from libMesh/stash_constraints

Stash constraints