MINOR: mworker/cli: split mworker_cli_proxy_create

MINOR: mworker/cli: split mworker_cli_proxy_create

There are two parts in mworker_cli_proxy_create(): allocating and setting up
MASTER proxy and allocating and setting up servers on ipc_fd[0] of the
sockpairs shared with workers.

So, let's split mworker_cli_proxy_create() into two functions respectively.
Each of them takes **errmsg as an argument to write an error message, which may
be triggered by some subcalls. The content of this errmsg will allow to extend
the final alert message shown to user, if these new functions will fail.

The main goals of this split is to allow to move these two parts independantly
in future and makes the code of haproxy initialization in haproxy.c more
transparent.