Adapted from Boost::filesystem.
Can you return an ErrorOr or Expected?
Use ErrorOr in the API
LGTM with nits.
Put STATVFS_F_FRSIZE(Vfs) in a variable. It is used 3 times.
git-clang-format the patch.
Avail, Total, Free.
Is there a reason we need this API?
clang-format (note: git clang-format does not run on .inc, I had to trick it)
Add two notes in the doxygen for Aaron comment (TOUTOC and windows quotas).
const Twine Path -> const Twine &Path?
Ignore this comment -- I misread it.
r265068 (and r265074 for the Twine reference)