reservation-mode is not global

I tried to set reservation-mode: global at the global scope in the MA output but the Kea config checker failed on the few generated configs with global reservations.

The reservation-mode keyword is allowed only in a shared-network or a subnet. This made sense before the global mode was introduced but not now.

IMHO it is a bug, not critical has global reservations are not yet heavily used. BTW it is easy to fix (one line in .ll, one in .yy, inheritance setup in 2 lines, a test) * 2 (v4 and v6).