Add support for Mac OS

[aszlig]

This is a WIP branch to implement support for Mac OS X, which in theory should work since it has a similar mechanism (DYLD_INSERT_LIBRARIES) to LD_PRELOAD on GNU/Linux.

Fundamental issues:

• [ ] Find a way to handle PID/GID in LOCAL_PEERCRED. It seems that there is only xucred.cr_uid, but in order to provide fake client IPs we need at least the peer PID to properly distinguish the individual peers.
• [x] We can't use std::filesystem yet, because Ubuntu 18.04 is still using GCC 7. Find a less ugly way to get_current_dir_name in a cross-platform way. According to https://ubuntu.com/18-04, the standard support has expired on 31 May 2023.

[aszlig]

For the record and according to https://github.com/golang/go/issues/27613, there seems to be a LOCAL_PEERPID socket option that we could possibly use.