More work towards getting bash to build.

Implemented some syscalls: dup(), dup2(), getdtablesize().
FileHandle is now a retainable, since that's needed for dup()'ed fd's.
I didn't really test any of this beyond a basic smoke check.

Kernel/Syscall.h

@@ -53,6 +53,9 @@ enum Function {
     PosixGetegid = 0x2021,
     PosixSignal = 0x2022,
     PosixIsatty = 0x2023,
+    Getdtablesize = 0x2024,
+    Dup = 0x2025,
+    Dup2 = 0x2026,
 };
 
 void initialize();