MFC r274785: Partially reconstruct Active/Standby clusting.

In this mode one head is in Active state, supporting all commands, while
another is in Standby state, supporting only minimal LUN discovery subset.

It is still incomplete since Standby state requires reservation support,
which is impossible to do right without having interlink between heads.
But it allows to run some basic experiments.
MFC r274795:
Close race between cfiscsi_offline() and new connection arrival.

Incoming connection should be either rejected or accepted and terminated.
MFC r274805:
Make cfiscsi_offline() synchronous, waiting for connections termination
before return.  This should make ctld restart more clean and predictable.
MFC r274248: Remove unused assignments, noticed by Clang analyzer.