Ensure people use xss-lock -l

When using xss-lock, the -l flag should be used so that the screen is locked _before_ the machine is suspended. Otherwise, at least on some machines, there’s a race where users can interact with the running applications (e.g. enter “ls” into a terminal) before the screen is locked.

We should:
* Update the manpage to not only talk about xautolock, but also xss-lock, and mention the -l flag there.
* In i3lock, check if the parent process is xss-lock, and if its command line is lacking -l, then print a warning. Need to double-check if said warning would actually be printed.