Constify prompt argument to read_command_lines

Constify prompt argument to read_command_lines

The prompt argument to read_command_lines can be const.  This patch
makes this change, and also removes some fixed-sized buffers in favor
of using string_printf.

ChangeLog
2018-05-04  Tom Tromey  <tom@tromey.com>

	* tracepoint.c (actions_command): Update.
	* cli/cli-script.h (read_command_lines): Update.
	* cli/cli-script.c (read_command_lines): Constify prompt_arg.
	(MAX_TMPBUF): Remove define.
	(define_command): Use string_printf.
	(document_command): Likewise.
	* breakpoint.c (commands_command_1): Update.