CVE-2026-16730: bump RLIMIT_NOFILE to the hard limit at startup. This raises the
session bus file-descriptor headroom from 1024 to 524288; the system unit sets a
single LimitNOFILE=16384, so soft equals hard there and the bump is a no-op.
accept4() in src/bus/listener.c still treats EMFILE/ENFILE as fatal, as upstream does.