(do_define): Keep info on source file of a symbol.
* coffgrok.h (coff_symbol): New field.
* srconv.c (PROGRAM_VERSION): Now 1.3
(wr_rl): Use external ref number for symbol.
(wr_dus): Only keep one source file per debug unit.
(wr_dln): Always emit line numbers for first source file,
(wr_globals): Emit globals in the du of their owning source file.
