slirp / vnet TFTP support:
Detect and reply all supported TFTP extension options for both read and write
operations. Use 'blksize' value if smaller than buffer size.
TODO: increase buffer size, implement timeout mechanism


git-svn-id: svn://svn.code.sf.net/p/bochs/code/trunk@11837 6c8e4198-a580-4521-a02f-c9ae86df1db9