Fix regression for multi breakpoints command line clearing.
    
    breakpoint.c is modified to fix the regression introduced
    when clearing the commands of several breakpoints by giving an empty
    list of commands, by just typing "end".
    GDB should read an empty list of command once, but it reads
    it for each breakpoint, as an empty list of command is NULL,
    and NULL is interpreted as 'not having read the command list yet'.
    
    The fix consists in having a boolean set to true once the
    command list has been read.
    
    gdb/ChangeLog
    
    2018-08-26  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
    
    	* breakpoint.c (commands_command_1): New boolean cmd_read
    	to detect cmd was already read.