Deprecate S.ieMode and rename it to something more indicative of what it actually is.

The `S.ieMode` boolean is misleading. As I mentioned [here](https://groups.google.com/d/msg/liftweb/SftLoq4CMmo/d9QlToX2NO8J) on the mailing list, this flag appears to be primarily intended to make sure we render XML that the legacy IE's can understand. It shouldn't, as its name implies, be regarded as a general IE detection mechanic.

As such, the following should probably happen (as discussed in the thread linked above):

- [x] Expose the *true* browser detection flags from the current Req instance directly in S.
- [x] S.ieMode should be renamed to be something it actually is. Probably S.ieLegacyRendering or something of the sort.
- [x] We should map S.ieMode to the new name with a deprecation warning in 2.6, slated for removal in 3.0.
- [x] Add a ticket for removing S.ieMode in 3.0.
- [x] Add user agent detection helper for IE 10, 11.
- [x] Figure out why IE < 6 isn't counted as IE.
- [x] Update calcIEMode comments and affected.