use mkstemp instead of mkostemp in FileFd::Open()

FileFd currently supports no fileflags which would make sense to provide
via mkostemp, so we can just use mkstemp here which is a standard
function compared to glib extension mkostemp.

O_CREAT (Create) and O_TRUNC (Empty) are implied by O_EXCL, which is the
mode mkstemp uses by default. The file description is opened ReadWrite,
but that used to be the default for FileFd in the old times and not a
problem as the difference is needed by FileFd to decide in which way the
compressor pipeline needs to be created (if any).

Git-Dch: Ignore

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