use imsg to handle ALL kinds of IPC in gmid. This simplifies and shorten the code, and makes everything more uniform too.

this fixes a bug introduced with the prefork mechanics: every server process shared the same socket, and this would cause a race condition when multiple server processes asked for a script cgi being executed. This gives each server process its own socket to talk to the executor, so the race cannot happen.

this is to let the regression suite compile

Include gmid.h as first header in every file, as it then includes config.h (that defines _GNU_SOURCE for instance). Fix also a warning about unsigned vs signed const char pointers in openssl.

Using BUFSIZ in sbuf is not OK. It's variable, and in various places we assume that sbuf is 1024 (like handle_cgi_reply). We could patch those, but we aren't sure BUFSIZ is >= 1024! Let's keep the hardcoded number. (found by debugging on arch on amd64, where BUFSIZ is bigger)

Before we mmap(2) file for reading, and use a buffer to handle CGI scripts. Turns out, for sequential access over the whole mmap isn't better than our loop on read. This has also the additional advantage that we can use handle_cgi (now handle_copy) for both files and CGI, which is pretty cool. This also fixes a nasty bug where we could hang a connection forever, because we scheduled the wrong type of event (read on POLLOUT and write on POLLIN, it's the other way around!)