Add passwd(1) to TODO
Rename `brk' to `_brk' to avoid polluting the namespace
Add support for obsolete -NUM arguments in arg.h

Fix naming, _brk, _argc and _argv are reserved for the
implementation.

Thanks to nsz@port70.net for reporting this, the mail excerpt below.

"actually __.* and _[A-Z].* are reserved for any usage and _.* is
reserved for file scope identifiers"

We just rename them to brk_, argc_ and argv_.
Rename __ARG_H__ to ARG_H__

__* is reserved so do not use it.