It's been more than 20 years that I had regular interaction with the French police, they tended to be quite authoritarian, like the state they represent and that's not always pleasant. The term might be widespread and not the derogatory, in the sence that French policemen will call themselves 'flics' and people generally refer to them as 'flics' with a neutral meaning, that doesn't mean you should adress one with 'flic'. They expect you to show respect in how you adress them and 'flic' is not the polite word, just as communication works much better if you use 'vous' instead of 'tu'.