ungetc will put characters back into an _IOSTRG input stream

[Casey Leedom]

Description:
	Ungetc will put characters back into an _IOSTRG input stream.  This
	will cause a segmentation violation if the string is in read only
	memory (shared text segment).  For example:

		if (!some_option) {
			opt = getenv(...);
			some_option = opt ? opt : "some_string";
		}
		...
		sscanf(some_option, "some_format", ...);

	In some cases, some_option will refer to the constant string
	"some_string".  Since sscanf uses ungetc to back up over input
	it decides it hadn't wanted to look at yet, we write into the
	constant string "some_string".

Repeat-By:
	Use gcc to compile the following program without the
	-fwritable-strings option:

	main()
	{
		int i, j;
	
		sscanf("4x4", "%dx%d", &i, &j);
		printf("%d %d\n", i, j);
	}

Fix:
	Debatable.  I fixed ungetc, but who knows?  Maybe we do want to
	be able to do ungetc's on a string argument.  Currently _IOSTRG
	is only used internally for sscanf and sprintf, so changing ungetc
	doesn't break anything now.

	Another option would be to replace all uses of ungetc in doscan
	with fseek(iop, -1L, 1) but I don't think fseek works on _IOSTRG
	arguments and I think it's a lot more expensive to call.

	Another option is to separate out the IO stream backup function
	from putting a [possibly different] character back on the stream.
	But this involves either defining a new interface, or extending
	the definition of ungetc's interface to accept something like
	ungetc(__IOS_BACKUP);

	And Finally, we could have read only streams and simply have
	sscanf set that flag in addition to _IOSTRG.  But this would add
	extra processing for every putc, etc.

	*** lib/libc/stdio/ungetc.c-dist	Wed Mar 26 18:08:43 1986
	--- lib/libc/stdio/ungetc.c	Sun Feb 12 13:45:30 1989
	***************
	*** 18,24 ****
	  			return (EOF);
	  
	  	iop->_cnt++;
	! 	*--iop->_ptr = c;
	  
	  	return (c);
	  }
	--- 18,27 ----
	  			return (EOF);
	  
	  	iop->_cnt++;
	! 	if (iop->_flag & _IOSTRG)
	! 		--iop->_ptr;
	! 	else
	! 		*--iop->_ptr = c;
	  
	  	return (c);
	  }