MINOR: mworker/cli: rename mworker_cli_proxy_new_listener

MINOR: mworker/cli: rename mworker_cli_proxy_new_listener

This is the first commit in a series to add the support of 4 primary reload
use-cases for the new master-worker architecture:

1. Newly forked worker process dies before any reload, due to some errors in
   the configuration. Newly forked worker process crashes before any reload
   after sending its "READY" state to master.

2. Newly forked worker process dies due to some errors in the new
   configuration. This happens after reload, when this new configuration was
   supplied, so the previous worker process is still here.

3. Newly forked worker process crashes after sending its "READY" state to
   master due to some bugs. This happens after reload, so the previous worker
   process is still here.

4. Newly forked worker process has sent its "READY" state to master and starts
   to receive traffic. This happens after reload, the old worker hasn't
   terminated yet, as it is waiting on some idle connection and it crashes.

Let's rename in this commit mworker_cli_proxy_new_listener() to
mworker_cli_master_proxy_new_listener() to outline, that this function creates
"master-socket" bind conf and allocates a listener. This listener is attached
to the MASTER proxy and it's bound to the ipc_fd[0] of the sockpair,
inherited in master and in worker processes (master CLI sockpair).