Rename one of the two sendme.h files

We have a convention that, to avoid confusion, two directories should never have headers with the same name. Our rectify_include_paths.py tool takes advantage of this fact to find a canonical path for each header. But in 0.4.1, we now have two conflicting headers:

./src/core/or/sendme.h
./src/trunnel/sendme.h

Let's rename one of these.