Feb 17 (Reuters) - Kuwait-based National Aviation Services (NAS) said on Thursday it had bought about 12.1 million shares in Britain's John Menzies for 73.4 million pounds ($100 million), days after the airport services group rejected NAS' takeover offer. The shares were bought at 605 pence apiece by Agility Strategies, an affiliate marketing vs performance marketing of NAS, and represent roughly 13.2% of...