Keep a module directory's setgid bit, warn about a refused runner, and restart the runner when it leaves (hq ADR 0334 review)

A module's own directories need setgid so what is made beneath keeps its own group,
and a mode of 2750 refused the whole declaration while every apply dropped the bit:
a directory's mode now carries setgid (never setuid or sticky), set, kept and said.
A tool runner's account that is missing or root's failed the whole apply and dropped
the library's outcome: it is now a warning, said as runner:<module> or
runner:library:<name>. And a runner that leaves a group is taken out before the
resources apply, so its unit is restarted in that apply to lose it.

internal/declaration/declaration.go

@@ -133,7 +133,13 @@ func (d *Directory) validate(where string, _ bool) []string {
 	if d.Path == "" {
 		problems = append(problems, where+": a directory needs a path")
 	}
-	return append(problems, checkMode(where, d.Mode)...)
+	// A directory's mode may carry the setgid bit (2xxx), so what is made beneath it keeps its group (novox/hq ADR
+	// 0334 §1); never setuid or sticky, which mean nothing the mesh asks of a directory it owns.
+	mode := d.Mode
+	if len(mode) == 4 && mode[0] == '2' {
+		mode = "0" + mode[1:]
+	}
+	return append(problems, checkMode(where, mode)...)
 }
 
 // File is a file with literal content. The host renders nothing.