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-node/files/settings.json

@@ -1,3 +1,4 @@
 {
-  "capacity": "shanks=3,ace=2,g14=0,novox=0"
+  "capacity": "shanks=3,ace=2,g14=0,novox=0",
+  "cpu-cap": "shanks=50,g14=50,ace=75"
 }