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.
MFC r268395:
Pass correct command that should be aborted to ISPCTL_ABORT_CMD.

This makes XPT_ABORT to work for me on initiator side of isp(4).
Previous code was trying to abort the XPT_ABORT itself and failed.
MFC r272937: Fix r272936 build with old GCC.
MFC r275420:

 gpioctl: don't print the command line arguments.

PR:	  	195330
Submitted by:	Scott Ellis jumpnowtek at gmail.com
MFC r271796 (by will):

  Use 'cc' for the C compiler instead of 'gcc'.

  Sponsored by:	Spectra Logic
  MFSpectraBSD:	1079507 on 2014/07/24
MFC r275206:
Assert the state of the process lock and sigact mutex in
kern_sigprocmask() and reschedule_signals().