libarchive: open_zipped() does not need to check extensions for e.g. gzip

We only need to check for signature-less extensions,
currently only .lzma. The rest can be happily autodetected.

This fixes "zcat FILE_WITHOUT_GZ_EXT" case, among others.

Change-Id: I2d52e398e2422265ba2ef530f9b2edfb20affc6f
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>