various fixes to get master branch to compile and run on macOS

[perbu]

First three commits are just trying to get things to compile without warnings.

The last one is an overflow. No /proc on macOS. Falls back to sysconf(_SC_OPEN_MAX) returns LONG_MAX, which overflows the int and then assert(maxfd > 0) isn't true anymore.

I've built and ran make check on Linux afterwards. Seems to work.

[perbu]

I suspect that failure is because of a flaky test. I can't see the relevance.

[bsdphk]

Sorry about the delay in getting to this.

I have resolved to of the problems in a more general way, will get to the rest.