Use std::string in mi-main.c

Use std::string in mi-main.c

Change a couple of spots in mi-main.c to use std::string, and change
one place to use field_fmt.  This removes some cleanups.

gdb/ChangeLog
2017-09-29  Tom Tromey  <tom@tromey.com>

	* mi/mi-main.c (mi_execute_cli_command): Use std::string.
	(mi_execute_async_cli_command): Likewise.
	(mi_cmd_trace_frame_collected): Use field_fmt.