FILEIO: add more error checking to XFILE routines.

Functions should start using XFFILE routines (instead of XFILE), which
return error codes.
FILEIO: convert pj_cpfile and pj_copy_file to XFILE.
FILEIO: convert read_gulp and write_gulp to XFILE.
FILEIO: convert a3d to XFILE.
FILEIO: convert celpaste to XFILE.
FILEIO: convert cluster to XFILE.
FILEIO: convert config to XFILE.