e2fsprogs: create open() and stat() helpers

e2fsprogs: create open() and stat() helpers

In many places we are using #ifdef HAVE_OPEN64 to determine if we can
use open64() but that's ugly. This commit creates two new helpers
ext2fs_open_file() for open() and ext2fs_stat() for stat(). Also we need
new typedef ext2fs_struct_stat for struct stat.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>