MDEV-18400 - Move shutdown handling to main thread

Signal handler is now respoinsible for setting abort_loop and breaking
poll() in main thread. The rest is handled by main thread itself.

Removed redundant LOCK_error_log init/destroy wrappers.
Removed redundant unireg_end(): it is trivial and it has only one caller.
Removed unused ready_to_exit from PFS.
Removed kill_in_progress: duplicates abort_loop.
Removed shutdown_in_progress: duplicates abort_loop.
Removed ready_to_exit: was used to make sure main thread waits for
cleanups, which are now done by main thread itself.
Removed SIGNALS_DONT_BREAK_READ, MAYBE_BROKEN_SYSCALL,
kill_broken_server: never defined/used.
Make clean_up() static.

sql/init.h

@@ -17,6 +17,5 @@
 #define INIT_INCLUDED
 
 void unireg_init(ulong options);
-void unireg_end(void);
 
 #endif /* INIT_INCLUDED */