Split trivial-httpd into separate binary

Split trivial-httpd into separate binary

Working on the libcurl backend, I hit the issue that the trivial-httpd program
depends on libsoup. I briefly considered having two versions, but libcurl is
client only, and moreover trivial-httpd is no longer trivial - it has various
features which are used by the test suite extensively.

Hence, what we'll do is build it as a separate binary which links to libsoup,
and use it during the tests. We *also* currently still provide `ostree
trivial-httpd` since some things use it like `rpm-ostree-toolbox` and the
Cockpit tests.

After those are ported to use some other webserver, I plan to add a build-time
option to drop it.

Closes: #636
Approved by: jlebon