i386: Remove two exported symbols that don't exist anymore

i386: Add symbols we need for the IRQ handler changes

This should have been committed in a previous changeset

kernel: Fix handling of stray IRQ's, these wouldn't correctly update the nested IRQ count

The result is a deadlock in Bochs, since it emulates stray IRQ #7's.

i386: Implement incrementing of the 'nested IRQ' per-CPU variable in IRQ handlers

kernel: Introduce a per-CPU 'nested IRQ' variable, which notes how deeply we are in IRQ handlers

This is necessary because interrupts may be nested, but only the final IRQ should trigger a reschedule action; otherwise, the original IRQ handler may not finish if the thread was suspended, as typically occurs during I/O.

kernel: Add a (mostly i386) gdb script which defines commands to inspect useful things

This makes it much easier to inspect threads and their contexts, among other things.

i386: Switch to the idle thread upon startup

The main reason is that this eliminates the 'curthread is NULL' case, which simplies the code. Another benefit is that we can use the idle thread's stack (which is larger than our bootstrap stack)

kernel: Add a function to resolve an address to a symbol

kernel: Fix a dumb oversight in the ELF64 code

kernel: Enable extra assertions in the scheduler code

These are on for now, at a performance hit - once the code has proved itself, they will be off by default.