tunables: sort tunables list (BZ 30027)

Sort tunables list at the time it's generated.  Note: adding new
tunables will cause other tunable IDs to change, but that was
the case before anyway.  POSIX does not guarantee the order of "foo
in bar" AWK operators, so the order was indeterminate before anyway.
Even depending on the order to be the same across multiple calls,
such as in this script, is undefined, so sorting the list resolves
that also.

Note that sorting is not dependent on the user's locale.