C++ify mi_parse

C++ify mi_parse

This changes mi_parse to return a unique_ptr, and to use "new"; then
fixes up the users.  This allows removing one cleanup.

gdb/ChangeLog
2017-04-12  Tom Tromey  <tom@tromey.com>

	* mi/mi-parse.h (struct mi_parse): Add constructor, destructor.
	(mi_parse): Update return type.
	(mi_parse_free): Remove.
	* mi/mi-parse.c (mi_parse::mi_parse): New constructor.
	(mi_parse::~mi_parse): Rename from mi_parse_free.
	(mi_parse_cleanup): Remove.
	(mi_parse): Return a unique_ptr.  Use new.
	* mi/mi-main.c (mi_execute_command): Update.