Let's not leak details from src/shared and src/libsystemd into
src/basic, even though you can't actually do anything useful with
just forward declarations from src/shared.
The sd-forward.h header is put in src/libsystemd/sd-common as we
don't have a directory for shared internal headers for libsystemd
yet.
Let's also rename forward.h to basic-forward.h to keep things
self-explanatory.
Let's move the operator enum into its own .c/.h file, so that we can
reuse it elsewhere, in particular systemd-analyze's compare-versions
logic.
Let's rename the concept CompareOperator, since it is nowadays
genericlaly about both order *and* fnmatch comparisons, hence just
naming it "order" is misleading.