Rename fd_io to file_io, and ReadFD to ReadFile, etc.
file_io.cc is the higher level versions that call the basic ReadFile/WriteFile
and then file_io_posix.cc and file_io_win.cc are the implementations of
those functions.
The Windows path is as yet untested, lacking the ability to link the test binary.
R=cpu@chromium.org, mark@chromium.org
BUG=crashpad:1
Review URL: https://codereview.chromium.org/811823003