ssh kitten: Support system where the login shell is a non-POSIX shell

Now the kitten passes its script to /bin/sh and detects the login shell
using various strategies based on the value of $USER on the remote
machine. The strategies are tried one after another till one succeeds:

1) getent passwd $USER
2) id -P
3) python pwd module
4) parsing /etc/passwd

The hope is that these strategies will fail on fewer systems than
relying on the login shell being POSIX compliant.

docs/faq.rst

@@ -55,7 +55,8 @@ type it each time::
     alias ssh="kitty +kitten ssh"
 
 If for some reason that does not work (typically because the server is using a
-non POSIX compliant shell), you can try using it with python instead::
+non POSIX compliant shell as ``/bin/sh``), you can try using it with ``python``
+instead::
 
     kitty +kitten ssh use-python myserver