Linker tables for automatic structures

Such as devtab, etc.
Fix devtab assumption with c->type

Inferno assumed devroot was the first dev in devtab.  They needed
certain chans (ones not associated with a dev yet) to not execute a
dev's close method.  rootclose() was a noop, so they managed.
Devtab created with linker tables

One tricky thing is alignment.  Regardless of sizeof(struct dev), the
struct devs in the .devtab section are aligned to 32 bytes.