PARI_stack_check false positives

[Jeroen Demeyer <jdemeyer@cage.ugent.be>]

Hello pari-dev,

the PARI_stack_limit functionality can sometimes lead to false 
positives: PARI assumes that the stack is essentially empty when 
pari_init_opts() is called, that there is only one stack, that 
getrlimit(RLIMIT_STACK) is constant. These assumptions are not 
necessarily true.

In particular I received a report from John Cremona who was running Sage 
in a multi-threaded application. Then each thread has its own stack 
causing failures in the PARI_stack_limit checking. Note that PARI was 
not compiled with thread-local storage.

Personally, I think that this stack check should not be enabled by 
default. I guess the main motivation for this functionality was GP, and 
you could enable that check just in GP. What do you think?

Cheers,
Jeroen.