RESOLV: Add an internal function to read TTL from a DNS packet
    
    Related:
    https://fedorahosted.org/sssd/ticket/1884
    
    Adds an internal resolver function that reads the TTL for SRV records as
    specified by RFC-2181. Several internal c-ares definitions are used
    until c-ares contains a function that exposes all this information via a
    parsing function.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>