We noticed that system(NULL) has a special meaning.  This patch augments the
fileio protocol to support it.

Fortunately the current protocol's LENGTH field is strlen + 1, so it can never
legitimately be zero.  So I use that to indicate a NULL string is being passed
-- we don't have to presume a NULL pointer is all bits zero  :)

Tested with a modified libgloss for an m68k target. ok?

btw, this patch requires my testsuite patch for break on } to be applied.

nathan
-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::         CodeSourcery
nathan@codesourcery.com    ::     http://www.planetfall.pwp.blueyonder.co.uk