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.

cli/parse_test.go

@@ -139,13 +139,17 @@ func TestValidateFloatNonNegative(t *testing.T) {
 
 func TestValidateBool(t *testing.T) {
 	b := byLong["enable-dht"]
-	for _, ok := range []string{"true", "false", "yes", "no", "1", "0", "on", "off"} {
+	// aria2 accepts only true/false; the invented yes/no/1/0/on/off spellings
+	// were removed, so they must now fail validation.
+	for _, ok := range []string{"true", "false"} {
 		if err := validate(b, ok); err != nil {
 			t.Errorf("validate bool %q: %v", ok, err)
 		}
 	}
-	if err := validate(b, "flase"); err == nil {
-		t.Errorf("validate bool flase: want error")
+	for _, bad := range []string{"yes", "no", "1", "0", "on", "off", "flase"} {
+		if err := validate(b, bad); err == nil {
+			t.Errorf("validate bool %q: want error (only true/false accepted)", bad)
+		}
 	}
 }