for use in special cases only; normal threaded use should prefer mutex.

We want to initialize the scheduler before the filesystem, so we can't
depend on elf::program.

This avoids using separate protocols for setup and runtime.

Simpler than phys_cast sometimes.

I've also added an osv/ prefix to make sure we don't clash with system headers.

We probably should either adopt this globally or drop it, but I fear we'll run into
problems with the latter approach.