APIs for attaching adjoint DirichletBoundary

We don't actually *use* the APIs yet.
Added DofMap::enforce_adjoint_constraints_exactly
Re-refactoring DofMap::_dof_constraints

In the presence of rare sparse heterogeneous adjoint constraints it
makes sense to store constraint rhs values in a separate map.  For
ease of refactoring (and in hindsight potential efficiency gains)
let's do the same with primal constraint rhs values
Bugfixes for rhs evaluations
Bugfix in adjoint Dirichlet boundary resize
More, more robust Dirichlet boundary accessors
Starting to use adjoint BCs in sensitivity calcs
Disable constraints when configured to
DofMap::heterogeneously_constrain_adjoint_rhs()

Took me forever to figure out how best to apply jacobian-dependent
constraint equations when we've already thrown away our element
Jacobians.
Assorted bugfixes.  We compile again.
Apply heterogeneous adjoint constraints
Assorted bugfixes
Merge branch 'master' of ssh://github.com/libMesh/libmesh into adjoint_dirichlet

Apologies to Derek for not either rebasing this (which would have been
a hassle for Vikram, who's following this branch) or foregoing the
merge until pull request time (which would have been impossible for
me; some of the code I want to test this with is dependent on API
changes in master)
Bugfixes for p refinement/coarsening
Properly clear old constraints when recalculating

This fixes two bugs:

A bug in master, where we fail to clear constraint coefficients
properly when the last constraints are removed from a 1D problem
(basically impossible to trigger)

A bug in adjoint_dirichlet, where we fail to clear heterogenous
constraint rhsvalues properly, ever (destroying solutions)
Bugfix for adjoint_already_solved init

Or lack thereof...
We can initialize based on a non-closed vector
Compute adjoint dirichlet constraints

And a bugfix for a case when some qois have constraints and others
don't
Remove overzealous asserts, print all constraints
Bugfix parallel heterogenous adjoint constraints