check_files.pl: now able to sum the size of files (can do it using SSH) and search which file is bigger.

- Added a new argument "-S [warn,crit]" to show the sum of the size of selected
  files.
- Added a new argument "-s [warn,crit]" to search for selected files which one
  is bigger.
- Performance data suppport for these new checks.
- Added a new argument "-H <IP_ADDRESS>" to execute the ls command on a remote
  host using SSH (used for host that does not have all Perl dependencies). Only
  public-key auth is supported.