Standard I/O doesn't work well with non-blocking handles, so make sure any pipes are in blocking mode before launching child processes.
Fixes https://github.com/libsdl-org/SDL/issues/10998
We require stdbool.h in the build environment, so we might as well use the plain bool type.
If your environment doesn't have stdbool.h, this simple replacement will suffice:
typedef signed char bool;
Also marked the existing functions as unsafe, as they can cause crashes if used in multi-threaded applications.
As a bonus, since the new functions are hashtable based, hint environment lookups are much faster.
