Mirror D3 strategy for drag event suppression

-webkit-user-drag is not inherited, so it's ineffective
at preventing drags of image overlays (#1821). Instead,
always preventDefault on dragstart.

While here, preventDefault on selectstart as well (a
"belt-and-suspenders" approach).

Together, these changes bring Leaflet's strategy in line
with D3's:

https://github.com/mbostock/d3/blob/master/src/event/drag.js