[IPSEC]: Sync series - acquire insert

This introduces a feature similar to the one described in RFC 2367:
"
   ... the application needing an SA sends a PF_KEY
   SADB_ACQUIRE message down to the Key Engine, which then either
   returns an error or sends a similar SADB_ACQUIRE message up to one or
   more key management applications capable of creating such SAs.
   ...
   ...
   The third is where an application-layer consumer of security
   associations (e.g.  an OSPFv2 or RIPv2 daemon) needs a security
   association.

        Send an SADB_ACQUIRE message from a user process to the kernel.

        <base, address(SD), (address(P),) (identity(SD),) (sensitivity,)
          proposal>

        The kernel returns an SADB_ACQUIRE message to registered
          sockets.

        <base, address(SD), (address(P),) (identity(SD),) (sensitivity,)
          proposal>

        The user-level consumer waits for an SADB_UPDATE or SADB_ADD
        message for its particular type, and then can use that
        association by using SADB_GET messages.

 "
An app such as OSPF could then use ipsec KM to get keys

Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>