core: merge src/config, src/logging, src/posix-signals into src/

core: merge src/config, src/logging, src/posix-signals into src/

Some subdirectories of src/ encapsulate large chunks of functionality,
but src/config/, src/logging/, and src/posix-signals/ are really only
separated out because they used to be built into separate
sub-libraries that were needed either for test programs, or to prevent
circular dependencies. Since this is no longer relevant, simplify
things by moving their files back into the main source directory.