slirp: Handle DHCP requests for specific IP (Jan Kiszka)

slirp: Handle DHCP requests for specific IP (Jan Kiszka)

This adds proper handling of the ciaddr field as well as the "Requested
IP Address" option to slirp's DHCP server. If the client requests an
invalid or used IP, a NAK reply is sent, if it requests a specific but
valid IP, this is now respected.

NAK'ing invalid IPs is specifically useful when changing the slirp IP
range via '-net user,ip=...' while the client saved its previously used
address and tries to reacquire it. Now this will be NAK'ed and the
client will start a new discovery round.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7198 c046a42c-6fe2-441c-8c8c-71466251a162