Introduce target_fileio_fstat

Introduce target_fileio_fstat

This commit introduces a new target method target_fileio_fstat
which can be used to retrieve information about files opened with
target_fileio_open.

gdb/ChangeLog:

	* target.h (struct target_ops) <to_fileio_fstat>: New field.
	(target_fileio_fstat): New declaration.
	* target.c (target_fileio_fstat): New function.
	* inf-child.c (inf_child_fileio_fstat): Likewise.
	(inf_child_target): Initialize to_fileio_fstat.
	* remote.c (init_remote_ops): Likewise.