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/sync.sh

@@ -18,6 +18,7 @@ cp ../Userland/cat mnt/bin/cat
 cp ../Userland/uname mnt/bin/uname
 cp ../Userland/clear mnt/bin/clear
 cp ../Userland/tst mnt/bin/tst
+cp ../Userland/ft mnt/bin/ft
 cp ../Userland/mm mnt/bin/mm
 cp ../Userland/kill mnt/bin/kill
 cp ../Userland/tty mnt/bin/tty