Fix: Set thread stack size to ulimit soft value

Some libc don't honor the limit set for the stack size and use their own
empirically chosen static value. Detect this behavior by checking if the
current stack size is smally than the soft limit and in that case set
the pthread stack size to soft limit value.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>