Warnings: Track basic $-references

We must always stop at $'s in the input now.

Having stopped at $, we normally just output it and carry on.  Ie, we
only stop so we can do some inspection: as before, we do this variable
tracking as inspection before processing, rather entangled with
processing.

We can deal reasonably properly with ${ } and $( ).

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>