to resume a transfer, download the requested document into a temporary file
which we later rename. This avoids leaving half-completed files around in
case of a crash (it'll still leave a half-completed file, but with a hope-
fully non-conflicting name), and should reduce the need for human inter-
vention on ports-building machines.
The temporary file name for "foo/bar" is constructed by invoking mkstemps()
with the pattern "foo/.fetch.XXXXXX.bar"
Requested by: obrien
e9dc4f3b74