This change updates the Linux symbols to correspond to v4.13 of the
kernel and v2.26 of glibc. To get everything building correctly, newer
versions of the build tools are required. Instead of trying to get an
Ubuntu 17.10 docker image working, we just add Artful sources to
/etc/apt/sources.list to get a similar effect.

Now that the kernel UAPI is updated, FS_MAX_KEY_SIZE and fscrypt_key no
longer need to be manually declared, as they are now in the proper
kernel headers.

Change-Id: I5f14c99fd70b43dbb8b44e05f53e4e9f09c9a345
Reviewed-on: https://go-review.googlesource.com/61771Reviewed-by: Ian Lance Taylor <iant@golang.org>