dtrace by default

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

I'd like to enable dtrace by default on i386, amd64, and arm -- that
is, setting MKDTRACE=yes in make for the userland tools, and enabling
options KDTRACE_HOOKS in the kernel for the hooks.

The overhead of KDTRACE_HOOKS in the kernel is a predicted not-taken
branch in a handful of places.  I have not measured the speed impact
but I expect it to be very small.  All logic related to dtrace is
relegated to a module, so unless you load dtrace.kmod, there's
negligible memory impact.

The overhead of MKDTRACE=yes in the userland is about 15 MB of dtrace
tools and libraries on amd64.

The benefits are manifold: low-overhead flexible diagnostics for
performance and debugging issues in the field with no kernel rebuild
or reboot.  In the past few months while trying to help people debug
things, my first question has often been `do you have dtrace?'.

Objections?