lib/pull: Change fetcher to return O_TMPFILE

lib/pull: Change fetcher to return O_TMPFILE

A lot of the libostree code is honestly too complex for its
own good (this is mostly my fault).  The way we do HTTP writes
is still one of those.  The way the fetcher writes tempfiles,
then reads them back in is definitely one of those.

Now that we've dropped the "partial object" bits in:
https://github.com/ostreedev/ostree/pull/1176 i.e. commit
https://github.com/ostreedev/ostree/commit/0488b4870e80ef575d8b0edf6f2a9e5ad54bf4df
we can simplify things a lot more by having the fetcher
return an `O_TMPFILE` rather than a filename.

For trusted archive mirroring, we need to enable linking
in the tmpfiles directly.

Otherwise for at least content objects they're compressed, so we couldn't link
them in. For metadata, we need to do similar logic to what we have around
`mmap()` to only grab a tmpfile if the size is large enough.

Closes: #1252
Approved by: jlebon