Fix regression for multi breakpoints command line clearing.

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.