fix(server): close inherited fds after daemonizing to avoid pipe deadlock (supersedes #2314)

[brianmacy]

Summary

Fixes the 0%-CPU daemon deadlock in #1011.

The persistent cache server is lazily fork+exec'd by a compile-job client
and inherits that client's open file descriptors — including ninja's jobserver
pipe and the cmake ... | tee stdout pipe. Because the long-lived daemon keeps
those write-ends open, the writer end never observes EOF, and the build wedges
at 0% CPU (reliably reproducible under a ninja build that pipes through tee,
e.g. an ONNX Runtime link step).

Fix

After daemonizing — and before the server binds its socket — close all inherited
file descriptors >= 3 via the close_fds
crate.

• Scoped to the cache server (Command::InternalStartServer). sccache-dist
  has already constructed a reqwest client that owns runtime/epoll fds by the
  time it daemonizes, so blindly closing fds there would be unsound — it opts out.
• daemonize() now takes (Option<File>, bool): the error-log stderr redirect
  is folded into daemonize (configured via daemonix's Stdio) instead of a
  separate post-daemonize step, and the bool selects whether to run the sweep.
• Opt-outs: SCCACHE_NO_DAEMON=1 (foreground debug) skips daemonizing and
  the sweep; SCCACHE_NO_FD_HYGIENE=1 is an escape hatch that skips only the
  sweep. On Windows there is no daemon fork, so daemonize() just redirects stderr.

Test

Adds a Unix regression test that forks, runs close_open_fds(3, &[]) in the
child, and asserts an inherited pipe fd is closed (fcntl(F_GETFD) → EBADF)
while stderr (fd 2) survives the sweep.

This has also been running in our own CI at scale (a fork build carrying this
change) across all six release targets with no regressions, and it eliminates
the ONNX-link deadlock we hit consistently without it.

Credit

This finishes off @ntrrgc's #2314, rebased onto current main and adapted to
the daemonize → daemonix migration (hence the daemonize(Option<File>, bool)
shape and the close_fds 0.3.2 dependency). Co-authored accordingly.

Closes #1011. Supersedes #2314.