[PATCH 2/7] lib-sieve: Drop binary-header resource usage tracking

[PATCH 2/7] lib-sieve: Drop binary-header resource usage tracking

With the per-user sieve-rusage file now authoritative for personal
storages, the .svbin header no longer needs to carry CPU usage. Admin
storages (sieve_before/after, default location) cannot be re-uploaded
by the user, so cross-run tracking is unnecessary there; the runtime
sieve_max_cpu_time check still enforces the per-execution limit.

Move the in-memory cumulative carry-over from sbin->header.resource_usage
to a dedicated sbin->persisted_rusage substruct. The on-disk header
struct now solely represents the wire format and is zeroed on every
save; the in-memory carry-over no longer overloads it. Drop the now-dead
"resource usage" section from sieve_binary_dumper_run() (the on-disk
fields are always zero, so the section never produced output).

Gbp-Pq: Name 0002-lib-sieve-Drop-binary-header-resource-usage-tracking.patch