iovec support for netsock

[toffaletti]

adding readv and writev is relatively easy, but there are a few considerations.

1. should the other send/recv calls just be wrappers around readv/writev? (state-threads does it this way)
2. to make this really useful we need a evbuffer-like api that contains a list of buffers and ownership information. like evbuffer, adding file descriptors and mmap'ed memory would be nice. it would be able to provide iovecs for its list of buffers. in the case of file descriptors it will probably need to provide a list of iovecs multiple times. or maybe we can special case this with sendfile() or splice(), needs more thought.