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.cpp

@@ -5,6 +5,11 @@
 
 extern "C" {
 
+pid_t fork()
+{
+    return Syscall::invoke(Syscall::PosixFork);
+}
+
 uid_t getuid()
 {
     return Syscall::invoke(Syscall::PosixGetuid);