the SSL case, it is no different from the old _fetch_write(), but in the
non-SSL case it uses writev(2) to send the entire vector as a single
packet (provided it can fit in one packet). Implement _fetch_write()
and _fetch_putln() in terms of _fetch_writev().
This should improve performance in the non-SSL case (by reducing protocol
overhead) and solve the problem where too-smart-for-their-own-good
firewalls reject FTP packets that do not end in CRLF.
PR: bin/44123
Submitted by: fenner