Allow LIO_READV and LIO_WRITEV in lio_listio().

[tmunro]

Summary

Allow multiple vector IOs to be started with one system call. The aio_readv() and aio_writev() functions already used the same code as lio_listio() under the covers. This commit just exposes the formerly internal-only LIO_READV and LIO_WRITEV opcodes to user space.

The new opcode are only visible to user space if __BSD_VISIBLE is defined, being extensions to POSIX, just like the aio_readv() and aio_writev() functions.

The other obvious candidate is LIO_SYNC, but that involves answering some tricky questions about dependencies and is left for later.

I subscribed the reviewers of D27743 (aio_readv() and aio_writev()).

Test Plan

A simple kyua test is included.

[tmunro]

Reuploaded patch with -U99999, no change.

[asomers]

Don't forget to bump the date in the man page.

tests/sys/aio/lio_test.c

230

The ftruncate is not necessary, since Kyua will always run the test in a brand-new directory.

[kib]

lib/libc/sys/lio_listio.2

231

s/FreeeBSD/.Fx/