RFC: Update MINSIGSTKSZ and SIGSTKSZ

[Paul Eggert]

On 9/29/20 2:49 PM, H.J. Lu wrote:
> What value do you have in mind?

64*1024

> What if it is not big enough 20 years later?

Then we change it later. We'll have plenty of time.

> Constant SIGSTKSZ and MINSIGSTKSZ are from history.   They should be changed.

Feel free to start the ball rolling at the POSIX level. In the meantime let's 
keep conforming to POSIX, since programs depend on constant SIGSTKSZ now. 
Changing POSIX will give application developers plenty of notice; changing glibc 
now won't do that.