Set the default thread stack size

POSIX does not specify the stack size of a thread created with
pthread_create(). tox needs a lot of stack, more than musl libc's
default stack size of 80KB, but less than glibc's 8MB. It seems that
128KB is enough.
Use 256KiB as default stack size for new thread
Merge pull request #192 from doughdemon/stacksize

Set the default thread stack size