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/option.go

@@ -122,7 +122,7 @@ var options = []Opt{
 	{Long: "allow-overwrite", Kind: Bool, Default: "false", Help: "overwrite an existing file", Tag: Advanced},
 	{Long: "auto-file-renaming", Kind: Bool, Default: "true", Help: "rename file (.1, .2, ...) if it already exists", Tag: Advanced},
 	{Long: "human-readable", Kind: Bool, Default: "true", Help: "show sizes as Ki/Mi/Gi", Tag: Advanced},
-	{Long: "file-allocation", Short: 'a', Kind: Enum, Default: "prealloc", Enum: []string{"none", "prealloc", "trunc", "falloc"}, Help: "how to allocate disk space", Tag: Advanced},
+	{Long: "file-allocation", Short: 'a', Kind: Enum, Default: "prealloc", Enum: []string{"none", "prealloc", "trunc", "falloc"}, Help: "how to reserve disk space (prealloc and falloc are identical here)", Tag: Advanced},
 	{Long: "dry-run", Kind: Bool, Default: "false", Help: "check that the file is available but do not download it", Tag: Advanced},
 	{Long: "stop", Kind: Int, Default: "0", Min: 0, Help: "stop the program after N seconds (0 = off)", Tag: Advanced},
 	{Long: "disable-ipv6", Kind: Bool, Default: "false", Help: "disable IPv6 (force IPv4-only connections)", Tag: Advanced},