threading: set min thread stack size; set it early

threading: set min thread stack size; set it early

Multi-tenancy uses loader threads that initialize detection engines. During
this, esp the AC family of MPM implementations, there is significant stack
usage. In most OS' threads have a lower stack size by default. In Linux, when
using the Musl C library, a thread by default gets 128KiB.

This patch does 2 things:

1. it centralizes the handling of the `threading.stack-size`. It it is not
   longer handled by the runmodes, but called from the global initialization
   logic.

2. it sets a minimum per thread stack size of 512k, unless `threading.stack-size`
   is set.

Ticket: #6265.