test: fix udp multicast interface test
unix: fix uv_tcp_nodelay return value in case of error

Fixes #1102
fs: vectored IO API for filesystem read/write

This improves API consistency with uv_read and uv_write and may
improve efficiency for some uses. Vectored IO is merely emulated on
Windows, which lacks the requisite system calls.
unix: portable preadv/pwritev use

preadv and pwritev are called directly on BSD, or by system call if
available on Linux. On systems that do not define them, preadv and
pwritev are emulated.