limit the configurable default stack/guard size for threads

limit to 8MB/1MB, repectively. since the defaults cannot be reduced
once increased, excessively large settings would lead to an
unrecoverably broken state. this change is in preparation to allow
defaults to be increased via program headers at the linker level.

creation of threads that really need larger sizes needs to be done
with an explicit attribute.

src/thread/default_attr.c

@@ -1,4 +1,4 @@
 #include "pthread_impl.h"
 
-size_t __default_stacksize = DEFAULT_STACK_SIZE;
-size_t __default_guardsize = DEFAULT_GUARD_SIZE;
+unsigned __default_stacksize = DEFAULT_STACK_SIZE;
+unsigned __default_guardsize = DEFAULT_GUARD_SIZE;