Constify some commands in breakpoint.c

This also makes delete_command static; but now I wonder if it is used
in Insight and should not be touched.

gdb/ChangeLog
2017-09-27  Tom Tromey  <tom@tromey.com>

	* breakpoint.h (delete_command): Don't declare.
	* breakpoint.c (delete_command, enable_once_command)
	(enable_count_command, enable_delete_command, breakpoint_1)
	(maintenance_info_breakpoints, stopin_command, stopat_command)
	(delete_command, delete_trace_command, save_breakpoints)
	(save_breakpoints_command, save_tracepoints_command): Constify.