XDR data_stream method for long int
More -Wconversion fixes

This batch of fixes is incomplete - it greatly reduces the number of
warnings, and fixes a number of data type problems that would have
caused errors above a few hundred million elements, but we'll need
more changes and more support from some of the underlying libraries
before we have 64-bit clean I/O.
Merge branch 'master' of github.com:libMesh/libmesh