trivial-httpd: Daemonize better
trivial-httpd: Daemonize better

I was trying to debug `test-pull-c`, and typing `Ctrl-C` in gdb
ended up sending `SIGINT` to trivial-httpd as well, killing it.

Daemonize a bit more properly to avoid this. I also followed the standard
`/dev/null` guidelines.

Closes: #643
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree