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.go

@@ -41,12 +41,6 @@ func humanSize(n int64, human bool) string {
 	return fmt.Sprintf("%.0f%ciB", val, suffix)
 }
 
-// speed formats a bytes-per-second rate. The DL:/UL: fields show the bare
-// abbreviated size with no "/s" suffix.
-func speed(bytesPerSec int64, human bool) string {
-	return humanSize(bytesPerSec, human)
-}
-
 // secfmt renders a duration as "1h2m3s", appending each unit only when it is
 // nonzero (but still showing seconds when the whole input is 0):
 // 3600s->"1h", 120s->"2m", 3720s->"1h2m". A non-positive or absurd