utcnow — Get the current unixtime in a no-std context

Work in progress.

Supported platforms

• Dragonfly (untested)
• Emscripten (untested)
• FreeBSD
• iOS (untested)
• Linux
• MacOS
• NetBSD
• OpenBSD (untested)
• Redox (untested)
• WASI
• wasm32
• Windows