Mark some non-static functions as MA_API

Return proper result when fopen() fails.

Fix Visual Studio 6/2003 compatibility

Apparently `errno_t` doesn't exist here either?

That last issue is annoying: `unsigned __int64` cannot be converted
to `double`, but `signed __int64` can. I'm not sure if using the
signed type in these cases instead is actually safe.

This is a much more common problem in dr_flac.

Public issue https://github.com/dr-soft/miniaudio/issues/139

Some headers are only required for the device IO API. This commit moves
these headers to the device IO section that they're excluded when
MA_NO_DEVICE_IO is set.

Public issue https://github.com/dr-soft/miniaudio/issues/138

Public issue https://github.com/dr-soft/miniaudio/issues/138

Public issue https://github.com/dr-soft/miniaudio/issues/139