Add login exercise

subjects/login/README.md

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+## Console
+
+### Login
+
+![TRON: Legacy](tron.jpg)
+
+> In the movie _TRON: Legacy_, Linux is rebranded "SolarOS" in reference to [Solaris](<https://en.wikipedia.org/wiki/Solaris_(operating_system)>), another Unix OS
+
+---
+
+Start the VM to boot the previously installed Debian system.
+
+Do the following for both a user and the [superuser](https://en.wikipedia.org/wiki/Superuser) (`root`) :
+
+- Log in
+- Clear the console using the keyboard shortcut
+- Change the password to a space : `" "`
+- Show the command history using five keystrokes or less (using autocompletion)
+- Log out using the keyboard shortcut
+
+### Just numbers
+
+Login as [`root`](https://en.wikipedia.org/wiki/Superuser) on the third [Linux console](https://en.wikipedia.org/wiki/Linux_console).
+
+Check the Internet connectivity with the command `ping google.com`.
+After a few hops, interrupt the program with : <kbd>Ctrl</kbd> + <kbd>C</kbd>.
+
+Behind every name in a computer system there is a number (ID, index, address, etc) :
+
+- User identifier
+  - `root` → `0`
+  - `student` → `1000`
+- IP address
+  - google.com → 216.58.214.14 (quad-dotted notation) → `3627734542`
+  - tencent.com → 117.169.101.44 (quad-dotted notation) → `1974035756`
+- File inode
+  - `/etc/fstab` → `44696029`
+  - `.profile` → `59639363`
+- Port
+  - `HTTP` → `80`
+  - `HTTPS` → `443`
+- Process identifier
+  - `cron` → `254`
+
+Names exist because they are human readable, but behind the scenes they are converted into numbers, unique in their namespace :
+
+- A domain name can have several IP addresses, but an IP address can only identify one domain name
+- Several processes may have the same name, but a PID identifies a single process
+
+Find the commands to get :
+
+- the inode of a specific file
+- the current user ID
+- the PID of a program, for example `bash`