make it possible to use a different socket for testing icecream

If ICECC_TEST_SOCKET is set to a unix socket, it'll be used
for communication between the client and daemon. Allows testing
icecream without conflicting with a possible system one.
fix native environment cleanup

If it gets removed because of being too old, remove it also from the native_environments map.
make calling icecc-create-env in daemon non-blocking

Creating environment for Clang can take a couple of seconds, exceeding
the connection timeout to daemon, thus the first compilation with a new env
can cause all but one first compilations to time out. Also,
the timeout will be fatal with ICECC_TEST_SOCKET.
fix typo in a comment
fix short option for --name
debug message when the scheduler decides to delay a job
fix 'elligible' typo
fix shadowing warnings (-Wshadow)

The listcs one in a previous commit was a real error, so it seemed worth
checking. Only one needless variable here, but these changes make sense
anyway.