Improved arc.groups processing

arc1.arc updated:

• line "$precomp $compressed" in arc.group now denote $precomp if such group presents in current compression method and $compressed otherwise

This allows fine-grain definition of compression groups, for example:

$precomp
*.pdf
$precomp $compressed
*.zip
*.jpg
$compressed
*.arc

files in second group will be included in group $compressed in -m4 compression mode, but in $precomp in -max compression mode