modify_header()
  Modify email headers.
  
  This scanner can be used to modify email headers.
  
  Usage: modify_header('^Header_regexp:','Header_regexp: prefix',scanners)
     or: modify_header(...).onlyif('virname')
  
  Where: '^Header_regexp:' is a string, which defines regular expression
           to find for this header
         'Header_regexp: prefix' is a string, which defines new header
           data
         'virname' is a string, which defines any virus name.
           By default all virus names at least 1 character long: '.'
         There are some special pattern, which are replaced:
           %V ... virus name
           %S ... scanner name
           %L ... detected level