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.

VirtualFileSystem/CharacterDevice.cpp

@@ -4,7 +4,7 @@ CharacterDevice::~CharacterDevice()
 {
 }
 
-OwnPtr<FileHandle> CharacterDevice::open(int options)
+RetainPtr<FileHandle> CharacterDevice::open(int options)
 {
     return VirtualFileSystem::the().open(*this, options);
 }