core: only apply NonBlocking= to fds passed via socket activation

core: only apply NonBlocking= to fds passed via socket activation

Make sure to only apply the O_NONBLOCK flag to the fds passed via socket
activation.

Previously the flag was also applied to the fds which came from the fd store
but this was incorrect since services, after being restarted, expect that these
passed fds have their flags unchanged and can be reused as before.

The documentation was a bit unclear about this so clarify it.