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.

LibC/unistd.h

@@ -8,6 +8,7 @@ __BEGIN_DECLS
 extern char** environ;
 
 inline int getpagesize() { return 4096; }
+pid_t fork();
 pid_t getsid(pid_t);
 pid_t setsid();
 int setpgid(pid_t pid, pid_t pgid);