Remove target_fileio_close_cleanup

Remove target_fileio_close_cleanup

This removes target_fileio_close_cleanup in favor of a new RAII class.
The new class is similar to scoped_fd but calls
target_fileio_close_cleanup rather than close.

Regression tested by the buildbot.

gdb/ChangeLog
2018-03-17  Tom Tromey  <tom@tromey.com>

	* target.c (class scoped_target_fd): New.
	(target_fileio_close_cleanup): Remove.
	(target_fileio_read_alloc_1): Use scoped_target_fd.