stream_libarchive: fix seeking fallback

In commit 1199c1e3, we added checks to every libarchive API call to make
sure the archive was closed on ARCHIVE_FATAL - otherwise, libarchive
could endow us with free CVEs (such as it apparently happens when you
continue reading a rar archive that uses features not yet supported by
libarchive).

This broke the fallback for seeking in unseekable archive formats. Of
course libarchive won't tell us directly whether a format implementation
has seek support or not - and OF COURSE it returns ARCHIVE_FATAL if it
has no seek support. (The error string, which you can retrieve via API,
is actually more detailed, and also claims it's an "internal error". I
don't think so, libarchive.) Returning ARCHIVE_FATAL means we have to
assume free CVEs are ahead, and we have to close the archive. Which
breaks the fallback in a dumb way (we have no way of telling which of
those cases happened anyway).

Fix this by assuming that all seek errors are potentially due to lack of
seek support. If the seek call fails, reopen the archive, and set a flag
so the seek API is never tried again. (This means we can still skip
ahead for forward seeks, which is more efficient than skipping from the
start of the archive entry.)

Also fix an old typo in an error message.

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