Because the health check uses OS specific syscalls for determining the
available space on a disk that aren't available in JS/WASM builds, we
need to make sure we don't reference that code at all. Otherwise we
can't use parts of lnd as a library in projects that are being compiled
down to a WASM binary.
With this commit we rename the existing AvailableDiskSpace function to
its correct name AvailableDiskSpaceRatio as it only returns a ratio. We
then go ahead and add a new function that returns the actual number of
free bytes available on a file system.
This also fixes some comments and always returns an error instead of
panicking.
