Use sudo's method (and code) to close file descriptors instead of iterating over every possible file descritor. In issue 5391 we see that closing file descriptors can take a very long time: 4 seconds in this user's case. We now close file descriptors in the file descriptor server before fork(). Since the file descriptor server is single-threaded, this works and is safe. Sudo uses opendir() after fork() which could deadlock, which we don't want to risk (and is a more serious concern for us since we're multithreaded) so when session restoration is disabled we still close every file descriptor because that's the state of the art on macOS (sigh).

When exec fails show the command, why it failed, and if it failed because the file wasn't found, show the PATH that was searched

Make the file descriptor server a process group leader in its own process group so shells without job control (or any child process that catches SIGINT but doesn't create process groups) will not cause the server to terminate on SIGINT. Issue 4288.

Remove the HandleEarlySigChild SIGCHILD handler from the FD server. It was causing a hang (issue 4383) because the server was very ill-behaved when its child process died immediately. Now the server should actually spin up and handle the death of its child properly.

Clean up diagnostic logging about server connections at startup. Don't try to connect to orphaned servers that we know are not orphans because we're already connected to them.

Establish socket connection between client and server before fork()ing to remove sleep-retry loop after fork(). Nuke the whole sleep-retry loop.

Pass around an iTermFileDescriptorServerConnection instead of a triple of fd and two pids.

Kill the server when there’s a broken pipe.

Don’t leak unix-domain socket file descriptors. Close them when the server dies.

Send only a single message from fd server to client, containing both the master pty file descriptor and the child's pid. Use shared code for the path to the unix domain socket, which now goes under /var/tmp, because it will have a longer lifespan there.

Conflicts:
	sources/iTermApplicationDelegate.m

Move source into sources/ directory. Delete a few unused images in the process and fix a reference to an Aqua image in FindView

Increase warning levels and fix warnings. The only inexcusably disabled warning (possibly) is for comparisons of different signs, simply because there are so many of them, in large part because methods like -count and -length return unsigned integers. IMO, the proliferation of unsigned ints is more dangerous than turning off the warning.

Fix bug where, when a new session is created with a custom starting directory, a login shell was not used. The fix is to run login -fpl $USER iTerm --launch_shell. We modify main() to see the --login-shell flag and exec the shell adding a - before its argv0.