aio_*(2): mention ENOSYS under ERRORS

ENOSYS can occur if aio(4) is not loaded in the kernel. Document this
behavior so consumers on FreeBSD can better understand that this is a
possible scenario.

Clean up the manpages slightly while here:

• Sort ERRORS by errno(3).
• Use .Fx instead of FreeBSD.

MFC after: 2 weeks
Reviewed by: ziaee
PR: 190942
Differential Revision: https://reviews.freebsd.org/D49502