agent: default nomination mode is regular

I propose to set the default nomination mode to regular instead of aggressive, as suggested by RFC 8445.

This modification is also interesting for sipe, that until now used a side-effect that receiving TCP remote candidates automatically and "systematically" switched from aggressive (the default) to regular. But since commit 0b80cbba, this is not systematic any more (when TCP candidates are received late after the start of the connection check).