Replace start_rbreak_breakpoints and end_rbreak_breakpoints

Replace start_rbreak_breakpoints and end_rbreak_breakpoints

This replaces start_rbreak_breakpoints and end_rbreak_breakpoints with
a new scoped class.  This allows the removal of a cleanup.

This also fixes an earlier memory leak regression, by changing
"string" to be a std::string.

gdb/ChangeLog
2017-11-04  Tom Tromey  <tom@tromey.com>

	* breakpoint.c
	(scoped_rbreak_breakpoints::scoped_rbreak_breakpoints): Rename
	from start_rbreak_breakpoints.
	(scoped_rbreak_breakpoints): Rename from end_rbreak_breakpoints.
	* breakpoint.h (class scoped_rbreak_breakpoints): New.
	(start_rbreak_breakpoints, end_rbreak_breakpoints): Remove.
	* symtab.c (do_end_rbreak_breakpoints): Remove.
	(rbreak_command): Use scoped_rbreak_breakpoints, std::string.