Rewrite target_read_string
    
    This rewrites target_read_string in terms of read_string.
    
    gdb/ChangeLog
    2020-06-15  Tom Tromey  <tromey@adacore.com>
    
    	* valprint.c (read_string): Update comment.
    	* target.c (MIN): Remove.
    	(target_read_string): Rewrite.