readelf: Handle compressed sections and extend -z to cover -x and -p.

When printing a (non-string, non-data) section use uncompressed data when
possible. For dumping hex and string sections (-x and -p) -z will dump the
uncompressed data (otherwise the compressed data is dumped).

-z, --decompress           Show compression information for compressed
                           sections (when used with -S); decompress section
                           before dumping data (when used with -p or -x)

Includes test cases for ET_REL files using compressed relocation (target)
debug sections to test libdwfl transparent uncompression of sections.

Signed-off-by: Mark Wielaard <mjw@redhat.com>