WIP: minor improvements to DMPlex and DMLabel

Created by: haplav

• DMAddLabel: Reference the label.

  This is a clear case of referencing one PetscObject from another.

• add DMPlexIsDistributed

• add DMPlexIsInterpolated

• DMPlexMarkBoundaryFaces: Check DMPlex is interpolated.

  Moved PetscValidHeaderSpecific from DMPlexMarkBoundaryFaces_Internal to DMPlexMarkBoundaryFaces.

• DMLabelGetStratumIS: manpage update

• DMRemoveLabel: nullify output label if it is exclusive reference

• DMRemoveLabel: remove redundant DMHasLabel() call

• DMRemoveLabel: Lookup the label by itself or its name.

• DMDestroyLabelLinkList: Rename to DMDestroyLabelLinkList_Internal.

• tests for DMAddLabel, DMRemoveLabel, DMPlexIsDistributed, DMPlexIsInterpolated

This is a preparatory PR for the upcoming thorough PointSF test.