MFC: r267131, r267132, r267133, r268493, r268671

Use NULL instead of 0 (Patch by Sascha Wildner <saw at online.de> for Dragonfly)
Remove unnecessary semicolons (Patch by Sascha Wildner <saw at online.de> for Dragonfly)
Add support for arbitrary http requests [1]
Support EAGAIN in fetch_writev

Submitted by:	Alex Hornung <alex at alexhornung.com> [1]
Reviewed by:	des