Fix: handle negative (unlimited) system stack size limits

Fix: handle negative (unlimited) system stack size limits

This also changes the stack size selection policy to select
the largest of:

1) default pthread stack size (dictated by libc)
2) system soft limit
3) 2 MB

This is bounded by the system's hard limit on stack size.
If this limit is smaller than 2 MB, the default size mentionned
in pthread_create(3) for Linux, we warn the user that the daemons
may be unreliable and advise bumping this limit.

Note that is is most likely possible to operate the daemons with
way less than 2MB of stack space. However, this was not
extensively tested.

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