Add flush, unmap and FUA support

This change will send flushes using NBD_CMD_FLUSH, also propagating the Force Unit Access flag if the NBD server supports those operations.

We'll stick to handling SCSIOP_SYNCHRONIZE_CACHE. We won't handle SRB_FUNCTION_FLUSH since those requests will come in pairs.

Since we're now going to handle NBD flags, with minimal changes we can start handling TRIM/UNMAP requests as well.

cloudbase / wnbd