libc: add aio_read2() and aio_write2() functions

as wrappers around lio_listio(LIO_READ/WRITE | LIO_FOFFSET, &iocb, 1);

Suggested and reviewed by: jhb
Discussed with: asomers
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differrential revision: https://reviews.freebsd.org/D43448