debug-generator: add new generator

debug-generator can mask specific units if they are specified on the
kernel command line with systemd.mask=.

debug-generator can pull in debug-shell.service is systemd.debug-shell
is passed on the kernel command line.
gitignore: sort properly
main: honour rd.systemd.unit= only in the initrd, not the host
main: don't show help text anymore when we detect an unknown kernel cmdline value starting with "systemd."

As generators and other components started to maintain their own kernel
command line options this help text needed more and more exceptions and
wasn't complete anyway. Fixing that would leak more information about
specific generators into PID 1, which we should avoid.

Given that kernel cmdline handling traditionally doesn't generate errors
or show help texts, let's just remove the logic for it for systemd too.
update TODO
man: add new kernel command line switches to kernel-command-line(7)
stop complaining about unknown kernel cmdline options

Also stop warning about unknown kernel cmdline options in the various
tools, not just in PID 1
gitignore: hide dhcp6/icmp6-rs tests