build: rename directory "include" to "shared"

build: rename directory "include" to "shared"

Up to now, the "include" directory contained (only) header files that were
used project-wide by libs, core, clients, et al.

Since the directory now also contains a non-header file, the "include"
name is misleading. Instead of adding yet another directory that is
project-wide, with non-header-only content, rename the "include"
directory to "shared".