Implement new sysconf keys. Change the implenentation of
_SC_ASYNCHRONOUS_IO in preparation for the next set of changes.
Move some limits which had been in <sys/syslimits.h> to <limits.h> where
they belong. They had only ever been in syslimits.h to provide for the
kernel implementation of the CTL_USER MIB branch, which went away with
newsysctl years ago. (There is a #error in <sys/syslimits.h> which I
will downgrade in the next commit.)