Added basic write support to the Slirp TFTP server.
TODO #1: implement all features of the existing (vnet) TFTP support in Slirp.
TODO #2: make the slirp DHCP server work like the vnet one.
TODO #3: finally remove the slirp backend and rename the new one to "slirp".


git-svn-id: svn://svn.code.sf.net/p/bochs/code/trunk@12259 6c8e4198-a580-4521-a02f-c9ae86df1db9