[PATCH 1/7] lib-sieve: Add per-user sieve-rusage file for cumulative CPU tracking

[PATCH 1/7] lib-sieve: Add per-user sieve-rusage file for cumulative CPU tracking

Cumulative resource usage was previously stored in each compiled Sieve
binary's (.svbin) header. Because the binary file is keyed by script
name, a user could reset their CPU budget by uploading the same script
under a different name (PUTSCRIPT newname + SETACTIVE newname),
RENAMESCRIPT, or DELETESCRIPT followed by PUTSCRIPT under another
name. Each such operation produced a fresh .svbin with zero counter,
allowing the sieve_max_cpu_time limit to be bypassed indefinitely.

Track cumulative CPU per user instead, in <INBOX-namespace-root>/sieve-rusage.
The file persists across all script renames, deletions, and re-uploads,
since it is not tied to any particular script name. Tracking is enabled
only for personal storages where the user's INBOX namespace has a
filesystem path; admin/default storages are unaffected here.

The on-disk format is a single ASCII line:

    V1 <flags> <cpu-secs> <update-time>

CPU time is rounded up to whole seconds when persisted; sub-second
precision is preserved only for the in-memory accumulation within a
single execution. The file is updated by writing a new copy via
file_dotlock_open_mode() and replacing the original via
file_dotlock_replace(). Concurrent writers are serialized through the
dotlock; readers do not need a lock because the rename is atomic.

The .svbin header still carries its original resource_usage struct as
the active fallback when no per-user file is available; a follow-up
commit drops that path.

Gbp-Pq: Name 0001-lib-sieve-Add-per-user-sieve-rusage-file-for-cumulat.patch