Fix #53467: Phar cannot compress large archives

When Phars are flushed, a new temporary file is created for each entry
which should be compressed, and the `compressed_filesize` is retrieved.
Afterwards, the Phar manifest is written, and only after that the files
are copied to the actual Phar.  So for each such entry there is an open
temp file, what easily exceeds the limit.

Therefore, we use a single temporary file for all entries, and store
the start offset in the otherwise unused `header_offset` member.  We
ensure that the `cfp` members are properly set to NULL even if flushing
fails, to avoid use after free scenarios.

This solution is based on a suggestion by @lserni[1].

Closes GH-6643.

[1] <https://github.com/box-project/box2/issues/80#issuecomment-77147371>

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