When acquiring a lease, record the value of the BOOTP siaddr field
contained in the DHCP offer, and write it out to the lease file
as an unquoted value of the "next-server" keyword. The value is ignored
when the lease is read back by dhclient, however other applications
are free to parse it.

The intent behind this change is to allow easier interoperability
with automated installation systems e.g. Cobbler, Foreman, Razor;
FreeBSD installation kernels can automatically probe the network
to discover deployment servers.  There are no plans to MFC this
change unless a backport is specifically requested.

The syntax of the "next-server <ip>" lease keyword is intended to be
identical to that used by the ISC DHCPD server in its configuration files.
The required defines are already present in dhclient but were unused before
this change. (Note: This is NOT the same as Option 66, tftp-server-name).

It has been exercised in a university protocol testbed environment, with
Cobbler and an mfsBSD image containing pc-sysinstall (driven by Cobbler
Cheetah templates). The SYSLINUX memdisk driver is used to boot mfsBSD.
Currently this approach requires that a dedicated system profile has
been created for the node where FreeBSD is to be deployed. If this
is not present, the pc-sysinstall wrapper will be unable to obtain
a node configuration. There is code in progress to allow mfsBSD images
to obtain the required hints from the memdisk environment by parsing
the MBFT ACPI chunk.  This is non-standard as it is not linked into
the platform's ACPI RSDT.

Reviewed by:	des