Drop support for Windows XP

Windows XP hit its end-of-life today (April 8, 2014).

We should identify and drop support code for Windows XP. This is mainly going to be a matter of identifying cases where we use LoadLibrary and GetProcAddress to find always-present-functions in always-present DLLs, and looking for opportunities to move from old busted APIs to fresh new ones.

I'm making this a separate ticket from #11444 (moved) (removing support from pre-XP versions) since the timing on the two can be argued to be separate. Nonetheless, if we agree to do both at once, that might be clever.