Add support for specifying a "usage" string for string based property definitions which define a pattern constraint. The usage string is used when the syntax of a property needs to be displayed, for example in a CLI help screen or in server-side log messages when a bad property value is encountered. For example, a property whose values must be a valid UNIX mode could have the following string syntax definition:

       <adm:string>
         <adm:pattern>
           <adm:regex>^[0-7][0-7][0-7]$</adm:regex>
           <adm:usage>MODE</adm:usage>
           <adm:synopsis>
             An valid UNIX mode string. The mode string must contain three
             digits between zero and seven.
           </adm:synopsis>
         </adm:pattern>
       </adm:string>

Existing string based properties have been updated with appropriate usage.