getdtablesize({2,3}) is a wonderful library call/system call to have access to because it allows one to determine the descriptor limits at runtime in an easier manner than having to do the equivalent with os.sysconf(..):

>>> os.sysconf(os.sysconf_names["SC_OPEN_MAX"])

This has been present in *BSD since time memorial [1] and in Linux since 2010 [2], so I think it would be a good addition to the `os` (`posix`) module.

I will submit a diff for this in a few days, if it's deemed acceptable to have in the `posix` module.

1. https://www.freebsd.org/cgi/man.cgi?query=getdtablesize&sektion=2&manpath=freebsd-release-ports
2. http://man7.org/linux/man-pages/man2/getdtablesize.2.html