Rework ACQ list functions.

Move all ACQ logic into ACQ releated functions. Add two hash tables
to speedup lookup. Make key_getacq() to return ACQ sequence number
instead of secacq structure. When we already have ACQ for given
secasindex, do not send SADB_ACQUIRE for each packet. Send SADB_ACQUIRE
only for first packet and then repeat it for each V_key_blockacq_count
packets. If ACQ doesn't exists, key_getacq() will use key_newacq().
Add key_acqreset() function to reset counter of blocked packets and
renew ACQ's TTL. Add key_acqdone() function to force ACQ expiration.