Wconversion fixes for dof_map_constraints.C
More -Wconversion fixes
Trivial change to save intel-12.0 from itself.

Seriously, this fixes a new issue:

  CXX    src/fe/libmesh_opt_la-fe_hierarchic.lo
src/error_estimation/kelly_error_estimator.C(140) (col. 45): internal error: 0_1451

compilation aborted for src/error_estimation/kelly_error_estimator.C (code 4)
make[1]: *** [src/error_estimation/libmesh_opt_la-kelly_error_estimator.lo] Error 1
make[1]: *** Waiting for unfinished jobs....

which must have been triggered by changes in the underlying header files,
as this file has not changed in ages.
Merge branch 'master' of github.com:libMesh/libmesh
More -Wconversion fixes

Mostly tedious trivial stuff, but this batch includes some more actual
"this would have broken at runtime above 2 billion dofs" fixes.
More -Wconversion fixes
More -Wconversion fixes
More -Wconversion fixes
More -Wconversion fixes

Lots of 64-bit correction fixes
Changes in gmsh_io.C for -Wshadow.

Issue #24.
Changes in gmv_io.C for -Wshadow.

Issue #24.
Changes in gnuplot_io.C for -Wshadow.

Issue #24.
Changes in matlab_io.C for -Wshadow.

Issue #24.
Changes in legacy_xdr_io.C for -Wshadow.

Issue #24.
Changes in medit_io.C for -Wshadow.

Issue #24.
Changes in mesh_communication.C for -Wshadow.

Issue #24.
Changes in mesh_data_tetgen_support.C for -Wshadow.

Issue #24.
Changes in mesh_data.C for -Wshadow.

Issue #24.
Changes in mesh_generation.C for -Wshadow.

Issue #24.
Changes in mesh_modification.C for -Wshadow.

Issue #24.