Implement basic chmod() syscall and /bin/chmod helper.

Only raw octal modes are supported right now.
This patch also changes mode_t from 32-bit to 16-bit to match the on-disk
type used by Ext2FS.

I also ran into EPERM being errno=0 which was confusing, so I inserted an
ESUCCESS in its place.

Kernel/SyntheticFileSystem.cpp

@@ -312,3 +312,9 @@ size_t SynthFSInode::directory_entry_count() const
     // NOTE: The 2 is for '.' and '..'
     return m_children.size() + 2;
 }
+
+bool SynthFSInode::chmod(mode_t, int& error)
+{
+    error = -EPERM;
+    return false;
+}