meson: default to -Dman=false to make development quicker

meson: default to -Dman=false to make development quicker

This makes the default build much quicker. If people are building systemd for
packaging or actual installation, they probably need to set some more options
anyway (-Ddns-servers=, -Dntp-servers=), so adding -Dman=true is not a big
burden.

For CIs configured locally, -Dman=true is added to restore status quo ante.