gdb: make watchpoint::val_valid a bool
    
    gdb/ChangeLog:
    
    	* breakpoint.h (struct watchpoint) <val_valid>: Change type to
    	bool.
    	* breakpoint.c (update_watchpoint): Assign false instead of 0,
    	true instead of 1.
    	(breakpoint_init_inferior): Likewise.
    	(watchpoint_check): Likewise.
    	(watch_command_1): Likewise.
    	(invalidate_bp_value_on_memory_change): Likewise.