Add support for the 'bootcmd' cloud-config directive, which allows
running commands very early in the boot process, before the hostname
is set and before the network is configured.

- nuageinit: bootcmd() function follows the same pattern as runcmd(),
  writing commands to /var/cache/nuageinit/bootcmds instead of runcmds.
  It is the first entry in the pre_network_calls table.

- rc.d/nuageinit: execute /var/cache/nuageinit/bootcmds immediately
  after /usr/libexec/nuageinit completes, before unmounting the config
  drive. This ensures bootcmd runs before NETWORKING per cloud-init spec.

On some environement a VM can be connected to multiple ifaces but only
one will respond to dhcp and able to fetch the metadata.

Launch the early dhclient on all available ifaces and as soon as one
dhcp has successfully configured an iface, proceed with fetching the
metadata.

MFC After:	1 week
Reported by:	mateusz.klejn@ovhcloud.com
Tested by:	mateusz.klejn@ovhcloud.com
Sponsored by:	OVHcloud

Sponsored by:	OVHCloud

Add naive support for openstack network config.
if no config driver have been found, try to detect that we are running a
VM on openstack via the smbios information, use the first iface
available and temporary activate dhcp on it, to be able to fetch the
necessary informations for cloudinit.

While here make the rc script execute after devmatch, some ethernet
device might be attached via devmatch and may be needed for cloudinit.

Tested on OVHCloud Public Cloud.

MFC after:	3 weeks
Sponsored by:	OVHCloud

Silence a warning emitted by fread(3) in fstyp(8)'s read_buf(), when
detecting the file system type of the cloud-init device:

    % fstyp /dev/iso9660/cidata
    fstyp: fread: Invalid argument
    cd9660

Also rephrase slightly a comment while here.

Signed-off-by: Jose Luis Duran <jlduran@gmail.com>

It prevents the home directory for the new users to be hidden
by late mount of the home directory

Reported by:	Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>

this is a very early script to support cloudinit, it does not intend to
be a full featured cloudinit client, but will support a good enough
subset to be viable in most case.

It support nocloud and openstack config-2 config drive mode (iso9660 or
msdosfs)

The following features are currently supported:
- adding users (including a default user named 'freebsd' with password
  'freebsd'
- adding groups
- adding ssh keys
- static ipv4, static ipv6, dynamic ipv4

With this one is able to use the 'bring your own image feature" out of
box.

It is expected that the script grows the support of other clouds
supporting cloud-init, contributions are welcomed.

It is designed to be only run once via the firstboot mecanism.

Sponsored by:	OVHCloud
MFC After:	3 weeks
Differential Revision:	https://reviews.freebsd.org/D44141