Warnings: Track settings of variables

Warnings: Track settings of variables

Where the variable tracking does not involve &-escapes, or only
involves &-escapes which work like a prefix so the processor does not
have the whole variable name, we do this separately from processing.

This is simpler because we can look ahead more.  It also avoids
disturbing the processing logic (which needs to be precisely accurate,
unlike this variable tracking).

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