Fix precallback validators in DNS plugin
    
    DNS plugin contains several RR type record validators run in
    pre_callback which cannot be used as standard param validator
    as it needs more data and resources that standard validators
    provide. However, the precallback validators are not run for
    DNS records created by new structured options and thus an invalid
    value may slip in.
    
    This patch moves the execution of these precallback validators
    _after_ the processing of structured DNS options. It also cleans
    them up a little and makes them more robust.
    
    https://fedorahosted.org/freeipa/ticket/2550