Implement fork()!

This is quite cool! The syscall entry point plumbs the register dump
down to sys$fork(), which uses it to set up the child process's TSS
in order to resume execution right after the int 0x80 fork() call. :^)

This works pretty well, although there is some problem with the kernel
alias mappings used to clone the parent process's regions. If I disable
the MM::release_page_directory() code, there's no problem. Probably there's
a premature freeing of a physical page somehow.

Kernel/MemoryManager.h

@@ -40,6 +40,8 @@ private:
 struct Region : public Retainable<Region> {
     Region(LinearAddress, size_t, RetainPtr<Zone>&&, String&&);
     ~Region();
+
+    RetainPtr<Region> clone();
     LinearAddress linearAddress;
     size_t size { 0 };
     RetainPtr<Zone> zone;