re PR libfortran/20179 (cannot mix C and Fortran I/O)

	PR libfortran/20179
	* io/unix.c (is_preconnected): Add function to test if a stream
	corresponds to a preconnected unit.
	* io/io.h: Add prototype for is_preconnected.
	* io/transfer.c (data_transfer_init): Do not truncate
	preconnected units.

From-SVN: r104141