_set_remote() is supposed to find out if a remote server has been
specified on the command line, but previously it only checked for -s
and ignored --server, causing the completion code to connect to the
local server instead when it should have connected to the remote
server to get the data for the completions.
This makes the Zsh completions work out-of-the-box. I also moved
pulseaudio-zsh-completion.zsh to zsh/_pulseaudio to be in line with
the common naming convention of Zsh completion files.
