Give MPI documentation project a unique name.

`libs/mpi/build/Jamfile.v2` and `libs/mpi/doc/Jamfile.v2` had the same project
name, which was causing the documentation build to fail.

svn path=/trunk/libs/mpi/; revision=79974
Several minor bug fixes and updates

svn path=/trunk/boost/mpi/; revision=82524
Several minor bug fixes and updates

svn path=/trunk/libs/mpi/; revision=82525