* add configure check * change the way the headers are required (copied from tmux)

* we don't add a space before the lang anymore * we're more strict in CGI handling: the `invalid' CGI script now triggers a 42 CGI ERROR

This is a big change in how gmid handles I/O. Initially we used a hand-written loop over poll(2), that then was evolved into something powered by libevent basic API. This meant that there were a lot of small "asynchronous" function that did one step, eventually scheduling the re-execution, that called each others in a chain. The new implementation revolves completely around libevent' bufferevents. It's more clear, as everything is implemented around the client_read and client_write functions. There is still space for improvements, like adding timeouts for one, but it's solid enough to be committed as is and then further improved.

We can't use normal pipe(2)s with libevent in some cases. Switch to socketpair(2), which doesn't have the same problem. This has the drawback that it doesn't prevent the CGI script from reading stdout, for instance. (sockets are two-way, pipes only one-way)

add the reported request id if there's a mismatch and both the gai error and the errno value if getnameinfo fails.

otherwise landlock will refuse to enable itself and the logger process dies.

it's needed by bufferevent_read

This changes the fastcgi implementation from a blocking I/O to an async implementation on top of libevent' bufferevents. Should improve the responsiveness of gmid especially when using remote fastcgi applications.