Add DOF-loading routines to the public API

[chrisa]

libusdt's raw DOF-loading routines are useful in other contexts, like dynamically loading ustack helpers. They're not part of the public API though, and perhaps ought to be.

Unfortunately the core platform implementations are relatively inaccessible (in the dynamic linker on OSX, and in the drti.o stub on Illumos-like systems).

[sax]

This would be amazing.

I'm slowly getting into working on a ruby ustack helper, and it finally hit me why I'm running into the problems I'm seeing developing it as a gem. I'm poking around at your Perl helper to see if I can reverse engineer DOF loading into Ruby.