Logging: L_SIGHANDLER should be really safe.

Logging: L_SIGHANDLER should be really safe.

I have realized that many operations performed by the log stream code are
either unsafe to call from signal handlers, subject to races between the
main program and its signal handlers, or subject to deadlocks when the
ucwlib_lock is taken.

I have therefore changed the semantics of L_SIGHANDLER to provide only
the minimal logging service: All sighandler messages are now logged to
stderr (which usually follows the main log file) and they are limited
to 255 characters (so that no dynamic allocation is performed).