[Serusers] Extending usrloc module to allow to save User-Agent values [patch for review]

Folks, Attached please find a patch, which extends usrloc/registrar modules to save values from User-Agent field in REGISTER messages into the database. It would be nice to have it included into the next release. Please disregard hackish detection of User-Agent fields, I am currently working on extending parser to recognize that type of field. I'll post updated patch when finished. Also, there is a new unrelated function which can be used to retrieve full list of all currently registered users in all domains. I am using this function in new version of my nathelper modules to do periodical UDP "pinging" of all registered users to keep their NAT bindings alive. I would like to have it reviewed and included into the next release as well. Thanks! -Maxim