Let the constructor and destructor do this.  This means that all users
have to be converted to allocate PcmConvert dynamically.

Expect OnSocketReady() to cancel events.  If it returns false, the
SocketMonitor may be destructed already.  This fixes a use-after-free
bug in the "httpd" output plugin.

Keep the SocketMonitor registered.  This wrong return value was added
6 years ago in commit 72cf8dd8, andd
apparently, nobody ever noticed.

This missing piece probably never really hurt, because
HttpdClient::OnSocketClosed() would be called right after a socket
error, but it's better to be explicit about closing on error.

Allows removing a CommandResult::CLOSE check.

This catches lines within command lists as well.

Eliminates a temporary copy.