[compress] delegate compress_level validation to compress libraries

• make val_level internal api call optional
• keep lzo2 val_level around due to the specific nature of compress_level values
• add internal compress_lib_test to do a round-robin (compress/decompress) check with provided values
• drop unnecessary val_level checks around
• update internal compress API include file
• adjust api_knet_handle_compress test to use a known bad value for zlib validation

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>