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fdio: Expose glnx_regfile_copy_bytes(), rewrite: GNU style, POSIX errno
NOTE: This changes the error handling API of `glnx_loop_write()` to be "old school POSIX" instead of "systemd". In ostree in a few places we use `g_output_stream_splice()`. I thought this would use `splice()`, but actually it doesn't today. They also, if a cancellable is provided, end up dropping into `poll()` for every read and write. (In addition to copying data to/from userspace). My opinion on this is - for *local files* that's dumb. In the big picture, you really only need cancellation when copying gigabytes. Down the line, we could perhaps add a `glnx_copy_bytes_cancellable()` that only did that check e.g. every gigabyte of copied data. And when we do that we should use `g_cancellable_set_error_if_cancelled()` rather than a `poll()` with the regular file FD, since regular files are *always* readable and writable. For my use case with rpm-ostree though, we don't have gigabyte sized files, and seeing all of the `poll()` calls in strace is annoying. So let's have the non-cancellable file copying API that's modern and uses both reflink and `sendfile()` if available, in that order. My plan at some point once this is tested more is to migrate this code into GLib. Note that in order to keep our APIs consistent, I switched the systemd-imported code to "old school POSIX" error conventions. Otherwise we'd have *3* (POSIX, systemd, and GError) and particularly given the first two are easily confused, it'd be a recipe for bugs.
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