Move socket to the vitunes directory.

Before, it wouldn't be possible for two simultaneous users to start
vitunes and send messages through the socket.

Its path (defaults to ~/.vitunes/vitunes.sock), like the remaining ones,
can also be configured with the newly added -s command-line option.
Always use sizeof with the socket functions.

In preference of, manually calculating it or possibly using SUN_LEN.
All cases are valid, because sun_path must be NUL terminated.
Use strlcpy(3) to copy the socket path.

Imperative, now that the path (POSIX says its size is undefined) can be
user controlled. In case of truncation, set errno accordingly, which is
caught by the caller.