Optimize attribute look-up, mostly useful in "git grep" on a
project that does not use many attributes, by avoiding it when we
(should) know that the attributes are not defined in the first
place.
* nd/attr-optim:
attr: avoid heavy work when we know the specified attr is not defined
attr: do not attempt to expand when we know it's not a macro
attr.c: rename arg name attr_nr to avoid shadowing the global one