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.

Userland/ft.cpp

@@ -0,0 +1,14 @@
+#include <stdio.h>
+#include <unistd.h>
+
+int main(int argc, char** argv)
+{
+    printf("Testing fork()...\n");
+    pid_t pid = fork();
+    if (!pid) {
+        printf("child, pid=%d\n", getpid());
+    } else {
+        printf("parent, child pid=%d\n", pid);
+    }
+    return 0;
+}