Update: memory function tweaks, update thread code, and improve execute functions.

Update: memory function tweaks, update thread code, and improve execute functions.

Restructure the memory functions a little.
Add comments about realloc() potentially changing the pointer address.

A few of the thread type macros are missing semicolons.
Remove f_macro_thread_mutex_t_clear().
There are likely other thread types that cannot be set to 0, and they will be correct as I discover them.

Rename the thread condition block and unblock functions.
Add missing f_thread_detach() implementation.

The execute fnctio fll_execute_program() should use pid_t when returning a PID.
This is potentially different from int, so instead use a void * and cast to an int * or pid_t * as necessary.