CDRIVER-3823 ignore SIGPIPE on osx

[samantharitter]

MSG_NOSIGNAL, the flag that we currently use to ignore SIGPIPE on POSIX systems, is not supported on OSX. Instead, OSX offers the SO_NOSIGPIPE flag that can be set as a socket option before performing writes.

I had asked @kmahar to test a different solution for ignoring SIGPIPE:
signal(SIGPIPE, SIG_IGN);

Upon further research, this solution isn't ideal, because it ignores SIGPIPE for the entire program, including any user code running a level above the C driver. In the case that a user is interested in SIGPIPE for some reason, setting the handler to SIG_IGN like this would also capture SIGPIPE in user code. The SO_NOSIGPIPE solution is less intrusive for a library like ours.

[kmahar]

this fixes the crash! thanks for fixing this, should make our macOS Evergreen tests a lot more reliable 🙂

[kevinAlbs]

Awesome, nice investigative work! I'm surprised we hadn't hit this before.
And I agree that this is better as a socket option instead of process-wide, so we do not interfere with existing signal handlers.