trivial-httpd: Port mostly to fd-relative

trivial-httpd: Port mostly to fd-relative

We were seeing some weird potential memory corruption in this code
when using it for `rpm-ostree-toolbox installer`, which is almost
certainly not its fault, but let's use it as an excuse to port
(mostly) to fd-relative and away from GFile.

Dropping the last GFile use here is a bit tricky as it does have a
nice high level wrapper around inotify.

Closes: #512
Approved by: jlebon