exec: replace OOMAdjust= by OOMScoreAdjust= to follow new kernel interface

exec: replace OOMAdjust= by OOMScoreAdjust= to follow new kernel interface

This replaces OOMAdjust= by OOMScoreAdjust= in the config files,
breaking compatibility with older unit files. However, this keeps compat
with older kernels which lack the new OOM rework.