Clean up _MAGIC handling.
 [jmk] --rw-rw-r-- M 1428987 glenda sys 8621 Jan 28 12:56 sys/include/mach.h
	/n/sourcesdump/2006/0128/plan9/sys/include/mach.h:217,225 - 
	/n/sourcesdump/2006/0129/plan9/sys/include/mach.h:217,226
	  typedef struct Fhdr
	  {
	  	char	*name;		/* identifier of executable */
	- 	uchar	type;		/* file type - see codes above*/
	+ 	uchar	type;		/* file type - see codes above */
	  	uchar	hdrsz;		/* header size */
	- 	uchar	spare[2];
	+ 	uchar	_magic;		/* _MAGIC() magic */
	+ 	uchar	spare;
	  	long	magic;		/* magic number */
	  	uvlong	txtaddr;	/* text address */
	  	vlong	txtoff;		/* start of text in file */