protothreads: Push the macros through clang-format

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>
protothreads: Add a few Doxygen comments

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>
protothreads: Add PT_CALL()

This could be regarded as slightly odd... a means to call a protothread
such that the system is not protothreaded.

Good for reusing library code though.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>