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/Syscall.h

@@ -47,6 +47,7 @@ enum Function {
     PosixGetpgrp = 0x2015,
     PosixTcsetpgrp = 0x2016,
     PosixTcgetpgrp = 0x2017,
+    PosixFork = 0x2018,
 };
 
 void initialize();