Remove cleanups from execute_gdb_command

Remove cleanups from execute_gdb_command

This replaces a cleanup in execute_gdb_command with an instance of
std::string.

Testing showed that this originally missed a cleanup that was returned
by prevent_dont_repeat.  This version of the patch changes
prevent_dont_repeat to return a scoped_restore rather than a cleanup.

2017-01-10  Tom Tromey  <tom@tromey.com>

	* top.c (prevent_dont_repeat): Change return type.
	* python/python.c (execute_gdb_command): Use std::string.
	Update.
	* guile/guile.c (gdbscm_execute_gdb_command): Update.
	* command.h (prevent_dont_repeat): Change return type.
	* breakpoint.c (bpstat_do_actions_1): Update.