vchoi_fork

[svn-r13829] Some systems (only SGI Altix ProPack 4 discovered so far) doesn't return correct
file size from MPI_File_get_size.  Bypass this problem by replacing it with
stat.  Add an option --disable-mpi-size in configure to indicate this function
doesn't work properly.  Add a test in testpar/t_mpi.c, too.  If it returns wrong
file size, print out a warning.

Tested on kagiso (parallel) because already tested the same change to v1.6 on 
several platforms (kagiso, cobalt, copper, and sol).