v.net: make snapping to lines (-s) the default for op=connect

[mlennert]

v.net proposes a flag (-s) that allows to snap points to the network in a 'connect' operation, instead of creating new arcs between the points and the network. In a majority of cases the result of using the flag is, IMHO, more useful than not using it.

See #2358 for a discussion of what happens when -s is not set.

I, therefore, propose to inverse the behaviour by making snapping to network the default, and using a flag (such as '-a' for 'add arcs') to activate the creation of connecting arcs.

As this kind of module API change is only possible in a next major release, I propose to at least change the documentation to recommend the use of the '-s' flag.

Moritz