Acquire jobserver's token prior to opening the lockfile

Acquire jobserver's token prior to opening the lockfile

Previously goredo opened corresponding lockfiles for each target
simultaneously, leading to many opened file descriptors. Now it takes a
jobserver's token before that, so number of opened lockfiles correlates
with the jobserver's slots. One of the drawbacks is that we wait and
take the jobserver's token even if target was actually already done before.