mapcache_seed: Refactor seeder for robust multiprocessing and shutdown

This commit is a major overhaul of the mapcache_seed utility, addressing critical bugs in signal handling,
multiprocessing, and thread synchronization that could cause deadlocks or unresponsive behavior.

The key changes are:

  1. Robust Signal Handling and Shutdown:

  The Ctrl+C (SIGINT) handling has been completely rewritten to ensure a prompt and clean shutdown in all modes, fixing
  numerous deadlocks and race conditions.

   - For Multiprocessing:
     - pop_queue() now checks a sig_int_received flag at the start to ensure child processes exit immediately after
       finishing their current task.
     - push_queue()'s EINTR retry loop now also checks the signal flag, preventing the parent process from deadlocking on
       a full message queue after a signal has been received.

   - For Multithreading:
     - On SIGINT, the feed_worker now calls apr_queue_interrupt_all() to wake all blocked threads. pop_queue() correctly
       handles the resulting APR_EOF to stop the worker.
     - A race condition where a thread could start a new job just as a signal was received has been fixed by re-checking
       the signal flag after a work item is successfully popped from the queue.

   - Graceful vs. Urgent Shutdown:
     - The final loop in feed_worker() that sends STOP commands is now wrapped in an if(!sig_int_received) block. This
       ensures it only runs during a normal, graceful shutdown and is skipped on Ctrl+C, preventing a major deadlock.

  2. Multiprocessing Enhancements:

  The -p mode has been significantly improved for stability and correctness.

   - IPC-based Logging: A new, dedicated System V message queue (log_msqid) has been implemented for logging in
     multiprocessing mode. This decouples logging from the main work queue, improving performance and stability.
     log_thread_fn and seed_worker were updated to use this new mechanism.
   - `-p 1` Bugfix: A long-standing bug that caused the seeder to stall when using -p 1 has been fixed by changing all
     relevant logic guards from nprocesses > 1 to nprocesses >= 1.
   - IPC Creation: Message queues are now created using IPC_PRIVATE instead of ftok, which is more robust.

  3. Code Quality and Minor Fixes:

   - child_init is correctly called at start of each worker process after fork().
   - sig_int_received is now correctly typed as volatile sig_atomic_t.
   - memset() is now used to initialize structs before use.
   - The help text for the -p option has been clarified.

Co-authored with gemini-2.5-pro

Author: marisn