remove proc_forward_imsg since it's unused

convert most of gmid to the new imsg APIs Makes parsing and handling of imsgs simpler / clearer. only crypto.c is left as-is.

convert to use imsg_get_fd() since proc_forward_imsg() never forwards a file descriptor (it's never called actually) just use -1 there.

rework check

work around missing SOCK_NONBLOCK/CLOEXEC on macos

remove proc_ispeer() unused, and was dropped by other copies of proc.c; reduces the diff with httpd' proc.c.

move setproctitle/privsep_process earlier We don't always do privilege dropping (as we may start as unprivileged user), so set these two beforehand so when we skip privdrop we don't miss to set privsep_process and set the process' title.

parse the config file only once Don't have all the processes read gmid.conf. The parent needs to do that, and the will send the config to the children (already happening.) The other processes were reading the config anyway to figure out the user and the chroot (if enabled); make the parent pass additional flag to propagate that info. We dissociate a bit from the "usual" proc.c but it's a change worth having.

fix build of proc.c on !OpenBSD

rework the daemon to do fork+exec It uses the 'common' proc.c from various OpenBSD-daemons. gmid grew organically bit by bit and it was also the first place where I tried to implement privsep. It wasn't done very well, in fact the parent process (that retains root privileges) just fork()s a generation of servers, all sharing *exactly* the same address space. No good! Now, we fork() and re-exec() ourselves, so that each process has a fresh address space. Some features (require client ca for example) are temporarly disabled, will be fixed in subsequent commits. The "ge" program is also temporarly disabled as it needs tweaks to do privsep too.