lio_listio(2): Allow LIO_READV and LIO_WRITEV.

Allow multiple vector IOs to be started with one system call.
aio_readv() and aio_writev() already used these opcodes under the
covers. This commit makes them available to user space.

Being non-standard extensions, they're only visible if __BSD_VISIBLE is
defined, like the functions.

Reviewed by: asomers, kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D31627