br704736: handle deep context-local macros

br704736: handle deep context-local macros

The documentation says that constructs with %$...$foo can be used
to access macros from deeper in the context stack.  From what
I can tell, that has never actually worked, since we'd enter names
like %$foo into the context-local macro name table.  Instead, only
insert the tail of the macro name into the context-local table;
expand get_ctx to also return a pointer to the macro name proper;
this is rather straightforward since we'd usually save away that
name at the point get_ctx is called anyway.