UTIL: Add a libtevent libcurl wrapper
    
    Adds a request that enables the caller to issue an asynchronous request
    with libcurl. Currently only requests towards UNIX sockets are
    supported.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>