TC: void getdfree( unsigned char drive, struct dfree *dfreep )

	- prototype in dos.h

	- fills in structure with disk status information.

	struct dfree
	    {
	    unsigned df_avail;		- available clusters
	    unsigned df_total;		- total clusters
	    unsigned df_bsec;		- bytes per sector
	    unsigned df_sclus;		- sectors per cluster
	    };

	- drive is specified as A = 1, B = 2, C = 3, etc...
	- in event of error, df_sclus is set to -1.
	- MS C uses _dos_getdiskfree()
	- see	INT 21,36