All frontends will have to include <libsigrok/libsigrok.h> from now on.
This header includes proto.h and version.h, both installed from the
distribution into $INCLUDE/libsigrok/ as well.
The only dynamically changed header is now version.h, which has both
libsigrok and libtool compile-time versions in it.
We should generally use api.c for API related functions and put the other
functions (mostly hardware-specific low-level code) into other C file(s)
for better readability.