Keep scratch mod dirs out of the repo - #246

* Keep scratch mod dirs out of the repo

A lab run needs a -game dir under engine/, and a git add -A has now
swept one in twice. Removes engine/argsession, which should never have
been committed, and ignores engine/*/ with engine/argus/ negated so the
shipped mod dir stays tracked and nothing else under engine/ can follow
it in.

* Stop two lab tests reaching outside themselves

The lab suite fails roughly one run in ten on this box and took the
lab job down on #246. Two causes, both mine, both from the tests
rather than the code they cover.

backup::two_installs_outside_the_root_both_survive called
restore_backup, and install_paths() includes the rerelease Saved Games
copy, which lives under USERPROFILE and not under the test's temp
root. The test was writing to a real install on the developer's
machine every run - the exact class #222 was about - and racing
anything else that touched it. take_backup only reads, so it stays;
the assertion moved to the manifest, which is where the #215 key
collision actually shows.

The GUI POST guard read the bound port from a process-global that the
concurrent lib tests share. A security check should not depend on
hidden process state, so the port is a parameter now and route() is a
thin wrapper that supplies it.

Neither change touches behaviour. A residual flake remains: two of ten
full-suite runs still failed after both fixes and I could not catch the
failing test name again in eight further runs, so it is rarer than it
was and not yet root-caused.