used e.g. for initrd or modules. Moreover gzip handling of not
easily seekable files is buggy. Disable unknown file size for now. May
be inefficient but works.
* grub-core/io/gzio.c (test_header): Always retrieve the file size.
* grub-core/io/xzio.c (grub_xzio_open): Likewise.
(grub_file_seekable): New inline function.
* grub-core/io/gzio.c (test_header): Don't test end magic if file isn't
easily seekable.
(grub_gzio_open): Set not_easly_seekable.
* grub-core/fs/i386/pc/pxe.c (grub_pxefs_open): Set not_easily_seekable.
* grub-core/io/bufio.c (grub_bufio_open): Propagate not_easily_seekable.