Split statfs and statvfs. The former has been available forever, and the
latter is implemented in terms of the former. The implementation has
been moved into headers so that it can be used at low API levels.
There's no reason for any Android or Linux code to use statvfs rather
than statfs, but code that needs to build on Darwin too will want to use
statvfs because Darwin's statfs is very spartan.
Bug: https://github.com/android-ndk/ndk/issues/609
Test: treehugger
Change-Id: Icf3d5723a260099fddb2d9f902e3047b0f041647
