This is allowed in C99 and C++11, but apparently not in C89 and C++98.
Make it conform to the older standards, since we want the client API
header to be highly portable.
Add a client API, which is intended to be a stable API to get some rough
control over the player. Basically, it reflects what can be done with
input.conf commands or the old slavemode. It will replace the old
slavemode (and enable the implementation of a new slave protocol).