release number bumped to 846 by export.pike
release number bumped to 847 by export.pike
Runtime: Increase the default thread C stack size to 1MB.

The old default (256KB) is a bit too little on current 64-bit hardware.

This essentially reverts 78797d06 (aka src/threads.c:1.157).

Most likely fixes [bug 7061].
release number bumped to 848 by export.pike
release number bumped to 849 by export.pike