units: synchronize Makefile and presets settings

Enable reboot.target and disable exit.target: the first is used on normal
machines, the second only in containers, and the more general one
should be enabled by default.

Also fix the Makefile to match what preset-all does.

With this and the previous commits, doing "make instal DESTDIR=…" followed
by "systemctl preset-all --root=…" doesn't result in any changes.