Use the new MeshBase::get_boundary_info() API in libmesh.

Refs libMesh/libmesh#388.
Reduce the number of times we call get_boundary_info().

Generally clean up some code.

Refs libMesh/libmesh#388.
Allow users to override compiler settings

This is helpful when one uses distcc. If you configure libmesh with
CXX='mpicxx' and then you want to use distcc, you have to go back and
reconfigure libmesh. This allows people to supply the libmesh_CXX, libmesh_CC,
libmesh_F77 and libmesh_F90 variables, thus using distcc without libmesh
reconfigure.

Closes #4171
Removing unused macros

Probably results of copy and paste.

Refs #000.
Replacing macro with static const value

Making the name more descriptive. Removing multiple definitions of the
same macro.
Formatting according to the code standards

Refs #000
Using libMesh:invalid_uint for the case when the variable is not coupled.

Refs #000
9999999 is not the max for double

Refs #000
Removing a magic number

Refs #000
Adding a .mailmap file to help organize the output of 'git shortlog'

Refs #000
Merge pull request #4122 from jwpeterson/boundary_info

Use the new get_boundary_info() API in libmesh
Merge pull request #4172 from andrsd/distcc

Allow users to override compiler settings
Merge pull request #4173 from andrsd/magic-numbers

Magic numbers
Merge pull request #4192 from jwpeterson/mailmap

Adding a .mailmap file to help organize the output of 'git shortlog'
Squash some statiz analyzer warnings (#3829)
Squash some more static analyzer warnings (#3829)
Squash even more static analyzer warnings (#3829)
Code formatting (#3829)