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.

VirtualFileSystem/FileHandle.cpp

@@ -15,6 +15,19 @@ FileHandle::~FileHandle()
 {
 }
 
+OwnPtr<FileHandle> FileHandle::clone()
+{
+    auto handle = make<FileHandle>(m_vnode.copyRef());
+    if (!handle)
+        return nullptr;
+    handle->m_currentOffset = m_currentOffset;
+#ifdef SERENITY
+    handle->m_fd = m_fd;
+    handle->m_isBlocking = m_isBlocking;
+#endif
+    return handle;
+}
+
 #ifndef SERENITY
 bool additionWouldOverflow(Unix::off_t a, Unix::off_t b)
 {