This commit addresses multiple server shutdown problems observed on macOS,
Solaris, and FreeBSD:

1. Corrected a non-portable assumption where socket shutdown was expected
to wake up poll() with listening sockets in the main thread.

Use more robust self-pipe to wake up poll() by writing to the pipe's write
end.

2. Fixed a random crash on macOS from pthread_kill(signal_handler)
when the signal_handler was detached and the thread had already exited.

Use more robust `kill(getpid(), SIGTERM)` to wake up the signal handler
thread.

3. Made sure, that signal handler thread always exits once `abort_loop` is
set, and also calls `my_thread_end()` and clears `signal_thread_in_use`
when exiting.

This fixes warning "1 thread did not exit"  by `my_global_thread_end()`
seen on FreeBSD/macOS when the process is terminated via signal.

Additionally, the shutdown code underwent light refactoring
for better readability and maintainability:
- Modified `break_connect_loop()` to no longer wait for the main thread,
  aligning behavior with Windows (since 10.4).
- Removed dead code related to the unused `USE_ONE_SIGNAL_HAND`
  preprocessor constant.
- Eliminated support for `#ifndef HAVE_POLL` in `handle_connection_sockets`
  This code is also dead, since 10.4