Expose SSIZE_MAX to enable safe/portable read and write system calls

The results of read [1] and write [2] system calls are implementation-defined if the count parameter is larger than value of SSIZE_MAX constant defined in limits.h according to POSIX.1 standard.

So in order to make it possible to use read and write in a portable manner, SSIZE_MAX should be exposed on POSIX.1-conformant systems.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/read.html
[2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/write.html

[maxbla]

I realize this issue is rather old, but is there any platform where SSIZE_MAX is not equal to libc::ssize::max_value()?