all: cut bloat found in the pike audit

Dead, vestigial, and over-built code with no behavior change (except the
bool-vocabulary trim, a deliberate behavior tightening):

- httpdl: drop the vestigial sort+copy in validSegs (work-stealing was
  removed in 270812d, so segments always tile [0,total) ascending now);
  validate order in place, which also rejects a scrambled sidecar. Inline
  the endOff() accessor to s.end.
- cli: shrink the boolean vocabulary to true/false, which is all that's
  accepted. The invented yes/no/1/0/on/off spellings are gone, so e.g.
  --enable-dht=yes (or yes in got.conf) is now a usage error.
- download: remove the dead, never-called Stat.Done() method.
- progress: drop speed(), a pure alias of humanSize; call humanSize directly.
- main: replace the jobsHolder mutex box with atomic.Pointer[[]job] for the
  forced-exit session save; the capability stays, the lock and type go.
- bt: collapse isAddrInUse's X||X (missinggo.IsAddrInUse is that exact
  string match); drop the now-unused missinggo import (tidy -> indirect).
- option: note that prealloc and falloc are identical here.

progress/format_test.go

@@ -53,16 +53,6 @@ func TestSecfmt(t *testing.T) {
 	}
 }
 
-func TestSpeedNoSuffix(t *testing.T) {
-	// The DL:/UL: fields show the bare abbreviated size with no "/s".
-	if got := speed(1536, true); got != "1.5KiB" {
-		t.Errorf("speed(1536,true) = %q, want %q", got, "1.5KiB")
-	}
-	if got := speed(2048, false); got != "2048B" {
-		t.Errorf("speed(2048,false) = %q, want %q", got, "2048B")
-	}
-}
-
 func TestPercent(t *testing.T) {
 	if got := percent(50, 100); got != 50 {
 		t.Errorf("percent(50,100) = %d, want 50", got)