Remove Debian support in the Ubuntu trunk.

This kind of build is not currently tested and, in antipication of the systemd
promotion, is deprecated in both Debian and Ubuntu.
Remove /etc/init.d/zfs-mount in the Ubuntu branches.

The ZFS_MOUNT and ZFS_UMOUNT variables are therefore obsoleted in the
`/etc/default/zfs` file.
Add /etc/init/zpool-import.conf for Upstart.

Accomodate zfsonlinux/zfs#2780 by importing pools in an upstart job before
mountall starts. The device timeout logic, although not event driven, should
mitigate the recurring mpt2sas problem described in section 4.3 of the ZoL
mountall FAQ.

Add a new `/etc/default/zfs` option:

  * `ZFS_AUTOIMPORT_TIMEOUT`, which is the number of seconds to wait
  for pool member devices to appear before attempting import.

Closes: zfsonlinux/pkg-zfs#53
Closes: zfsonlinux/pkg-zfs#101 for Ubuntu.
Relax the /etc/hostid copy in the initramfs hook.

The hostid check was deprecated in ZoL 0.6.3 at upstream commit
zfsonlinux/spl@acf0ade362cb8b26d67770114ee6fa17816e6b65, which means that the
/etc/hostid file may not exist on all systems.