Avoid seqno mismatch oops in recursive land unit supply

Avoid seqno mismatch oops in recursive land unit supply

Before s_commod() attempts to recursively supply a supply unit it
wants to use as supply source, it zaps the unit's load.  When
actually_doit is false, it later restores the old load by overwriting
the change with a saved copy of the unit.  That triggers a seqno
mismatch oops.

Avoid that by copying the new sequence number to the saved copy.