Add login shell mode

[rkdarst]

• This will run with (for example with bash) bash -l -c kernel-cmd,
  so that we get a clean user shell. It could be combined with
  --norc to avoid user RC files.

• This requires adjusting tests some, since this mode works
  fundamentally differently: it sets the kernel to directly invoke
  bash -l, instead of invoking envkernel to do something.

  • Some extra login for splitting the envkernel 'ek' options from the
    kernel 'k' options.

  • Excluding loginshell from running on some of the tests, which
    fundamentally work differently.