[PATCH 3/7] lib-sieve: Rename on-disk header rusage fields to unused_*

[PATCH 3/7] lib-sieve: Rename on-disk header rusage fields to unused_*

The .svbin header's resource_usage fields no longer carry data: they
are zeroed on save and discarded on load. Cumulative CPU tracking
lives in the per-user sieve-rusage file, and the in-memory carry-over
lives in sbin->persisted_rusage.

Flatten the nested resource_usage struct and rename the fields to
unused_update_time and unused_cpu_time_msecs so it is obvious the
fields are kept only for on-disk format compatibility. The memory
layout of struct sieve_binary_header is unchanged.

Gbp-Pq: Name 0003-lib-sieve-Rename-on-disk-header-rusage-fields-to-unu.patch