[PLXCOMP-244] Don't try to set attributes of symbolic links

Symbolic links themselves don't have changeable file attributes. Trying to change attributes of target file is IMHO a bad idea.

Example 1: Symlink in archive is stored with attributes 777. Extracting such symlink with plexus-archiver will make target file world-writable!

Example 2: Symlink points to system file (owned by root). If ordinary user tries to extract such symlink with plexus-archiver then there will be `java.nio.file.FileSystemException: Operation not permitted` thrown as ordinary user cannot change attributes of file owned by root.

JIRA bug report: http://jira.codehaus.org/browse/PLXCOMP-244