systemd: bump to version 214

This patch bumps systemd to version 214.

Changes:

- add new users: systemd-bus-proxy, systemd-network and systemd-resolve.
- remove gtk-doc patch, as it is no longer needed.
- remove attr dependency (extended attributes functions are provided by
  glibc).
- enable nss-myhostname, so local hostname changes are properly handled.
- add hook to install symlink to resolv.conf when using
  systemd-networkd.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>