chg: dev: Shrunk cfgobj down from 48 bytes to 40 bytes

Follow-up of 38ce2906 as the size of the `cfg_obj_t` can actually goes
down to 40 bytes "for free", by using bitfields to only use 31 bits for
the `line` field, so the remaining bit can be use to hold the `cloned`
state without paying the extra 8 bytes padding.

Merge branch 'colin/cfgobj-40bytes' into 'main'

See merge request isc-projects/bind9!11334