[invirt-dhcp] Improve standards compliance

[invirt-dhcp] Improve standards compliance

Provide a server-identifier option containing our IP address, as
is required by several DHCP clients including gPXE (used for the
Debathena network install CD) and Windows.

Reuse the existing DHCP socket in a bindtodevice - sendto - restore
global binding sequence, instead of creating a new socket for each
reply, in order to keep replies coming from port 67. This is required
by the standard, some client programs, and all DHCP forwarders.

svn path=/trunk/packages/invirt-dhcp/; revision=2491