[kernel] Add kernel and user mode divide by zero handling

[ghaerr]

Adds handler for divide by zero and divide overflow hardware exception thrown in DIV or IDIV instructions.

Solves problem identified in Mellvik/TLVC#71 (comment).

If the fault occurs from a user application, a SIGABRT signal (ELKS has no SIGFPE) is sent, which usually causes program termination. For kernel faults, a divide by zero panic message is shown and the kernel stops executing. Restarting any code after a DIV exception turns out to be quite complicated, as the hardware fault pushes a different CS:IP depending on whether the CPU is prior to an 80286 or not.

Also removed/rewrote "IRQ allocation" scheme used in irq.c as overly complicated, which saves ~100 bytes in kernel. It is now easy to add any other hardware trap/fault handler to the kernel should that be wanted.

Removed intr_count global from irqtab.S as unneeded, the _gint_count variable already tracks kernel reentry.