Add a "prompt_login" helper function

This prints a description of the "host". Currently that's

`(chroot:debianchroot) $USER@$hostname`

with the chroot part when needed.

This also switches the default and terlar prompts to use it, the other
prompts have slightly different coloring or logic here.

doc_src/cmds/prompt_login.rst

@@ -0,0 +1,26 @@
+.. _cmd-prompt_login:
+
+prompt_login - describe the login suitable for prompt
+=====================================================
+
+Synopsis
+--------
+
+::
+
+    function fish_prompt
+        echo -n (prompt_login) (prompt_pwd) '$ '
+    end
+
+Description
+-----------
+
+``prompt_login`` is a function to describe the current login. It will show the user, the host and also whether the shell is running in a chroot (currently debian's debian_chroot is supported).
+
+Examples
+--------
+
+::
+
+    >_ prompt_login
+    root@bananablaster