Build modules: a per-node processor cap, and each step's threads within it (issue 1806)

The operator's desktop slows while it builds (shanks at 92%, the Go build
runtime at 1650% of 24 processors). The build modules declare cpu-cap
(default shanks=50,g14=50,ace=75), which the composer turns into the runtime's
quota at idle priority; each step's go, node and buildah commands get
GOMAXPROCS and UV_THREADPOOL_SIZE for their part of it, and the status tools
show the limits and what the kernel holds the runtime to.

modules/mesh-build-image/module.json

@@ -24,6 +24,12 @@
       "description": "How many asks of the image build seat this module runs at once on each node, as node=asks pairs separated by commas, such as shanks=3,ace=2 (novox/hq ADR 0525 §5); a node set to 0 pulls nothing, a node not named runs one at a time. The control-node and the laptop build nothing.",
       "pattern": "|[a-z0-9_-]+=[0-9]+(,[a-z0-9_-]+=[0-9]+)*",
       "default": "shanks=3,ace=2,g14=0,novox=0"
+    },
+    "cpu-cap": {
+      "type": "string",
+      "description": "The most of each node's processors this module's builds may take, as node=percent pairs separated by commas, such as shanks=50,ace=75 (novox/hq issue 1806); a node not named takes 50. The node-engine holds the module's runtime to it as a CPU quota, at idle priority, and each build step starts no more threads than its part of it.",
+      "pattern": "|[a-z0-9_-]+=([1-9]|[1-9][0-9]|100)%?(,[a-z0-9_-]+=([1-9]|[1-9][0-9]|100)%?)*",
+      "default": "shanks=50,g14=50,ace=75"
     }
   },
   "tools": [
@@ -77,7 +83,7 @@
       "path": "${dir:state}/settings.json",
       "mode": "0600",
       "owner": "mesh-build-image",
-      "takes-config": ["capacity"],
+      "takes-config": ["capacity", "cpu-cap"],
       "merge": "json",
       "template": "files/settings.json"
     },