Deflake exec test.

- Only use MAXSYMLINKS/2+1 symlinks for each of the interpreter and script
  paths in SymlinkLimitRefreshedForInterpreter to tolerate cases where the
  original paths (/tmp, /bin, or /bin/echo) themselves contain symlinks.

- Ensure that UnshareFiles performs execve immediately after clone(CLONE_VFORK)
  (no heap allocation for ExecveArray/RunfilesPath).

- Use lstat() rather than stat() for the existence check in fs_util's Exists;
  the latter will fail if the symlink target does not exist, even if the
  symlink does.

PiperOrigin-RevId: 320110156

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