Send locking assertion backtraces to stderr.  Improve the heading on the backtrace.
Send locking assertion backtraces to stderr.  Improve the heading on the backtrace.

stapelberg/libxcb,OpenInkpot-archive/iplinux-libxcb,stapelberg/libxcb,OpenInkpot-archive/iplinux-libxcb,topcat/xcb-win32,topcat/xcb-win32,topcat/xcb-win32,topcat/xcb-win32,stapelberg/libxcb,OpenInkpot-archive/iplinux-libxcb,topcat/xcb-win32