generator.ml: new type, "Pathname"

* src/generator.ml: Emit NEED_ROOT and ABS_PATH into generated
stubs.c, rather than requiring they be added manually at the start
of each and every do_* function that operates on a "path" parameter.
Update grammar: Pathname is just a String, with the above exception.
Do not update augeas "path" parameters, since they are not file names,
but rather ":"-separated search paths.  Except aug_init, for which
"path" *is* a file name.