Round-3 cold-boot trims:
- mask sysstat.service (sadc CPU/IO logger; nothing reads it).
- mask dphys-swapfile.service and add /var/swap to /etc/fstab so swap
is brought up by systemd at local-fs.target instead of by an LSB
wrapper that re-checks the swap file size on every boot.
Both are reversible: `systemctl unmask <unit>` and remove the fstab
line. Before doing the dphys swap, install.sh verifies /var/swap
exists; on a fresh image where the file hasn't been created yet,
nothing is changed and dphys-swapfile keeps running normally.
Userspace boot 11.5s -> 10.7-11.4s on clean runs; bbctrl listen
unchanged at boot+10.4s (the saving moves to chromium/multi-user).
Henrik Muehe
3d73e6c59d