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/options.go

@@ -27,13 +27,14 @@ func (o *Options) IsSet(name string) bool { return o.set[name] }
 // Str returns the raw string value (empty if unset and no default).
 func (o *Options) Str(name string) string { return o.vals[name] }
 
-// boolWords is the single accepted vocabulary for boolean options, mapping each
-// recognised spelling to its truth value. The Bool reader, truthy, and
-// validate() all consult it, so exactly the words that validate are honoured
-// (no "accepted by the reader but rejected by validate" surprises like --x=on).
+// boolWords is the accepted vocabulary for boolean options. aria2 takes only
+// true/false (and rejects everything else), so we match it rather than inventing
+// extra spellings. boolWord is the single consult point — the Bool reader,
+// validate(), and the no-conf bootstrap all go through it — so exactly the words
+// that validate are honoured.
 var boolWords = map[string]bool{
-	"true": true, "yes": true, "1": true, "on": true,
-	"false": false, "no": false, "0": false, "off": false,
+	"true":  true,
+	"false": false,
 }
 
 // boolWord reports a value's truth and whether it is a recognised boolean word.