Added descriptions of several new contributed files/directories

hostlist: factor out a pattern

This is a clerical patch which transforms a repeating pattern of type

 	if (dims > 1) {
		if (hr->width == dims) {
			// Alternative 'A'
		} else {
			// Alternative 'B'
		}
	} else {
		// Alternative 'B'
	}

into the shorter pattern

	if (dims > 1 && hr->width == dims) {
		// Alternative 'A'
	} else {
		// Alternative 'B'
	}

Miscellaneous typos, found while reading documentation.

The majority is in the manpages and within comments, only one occurrence
is in an error message (function name of set_node_down).

Allow compilation with --enable-cray.

This patch allows compilation of slurm with --enable-cray without build errors,
on such (multi-cluster) systems that need to be able to load select/cray, but
without actually using the internal functionality (e.g. slurmdbd host).

The code stubs have been translated into corresponding comments with FIXMEs
added, these are then addressed later in the (post-2.2) patch set.

Added the missing part (variables) to fix build errors and tested tested that 
the compilation of --enable-cray works without errors.

    a job was requeued and started no job steps.

Increase number of active threads in the slurmdbd from 50 to 100

    far in the future..

    CPUs on some nodes. Patch from Hongjia Cao, NUDT.

    new logic from sched/backfill and added pthread) and more accurate.

sched/builtin for better efficiency

    counter for larger job sizes. Patch from Hongjia Cao, NUDT.