install: Run GPM.

Suggested by Adam Pribyl <pribyl@lowlevel.cz>
at <https://lists.gnu.org/archive/html/guix-devel/2015-11/msg00133.html>.

* gnu/system/install.scm (installation-services): Add call to
  'gpm-service'.
* doc/guix.texi (System Installation): Mention GPM.

doc/guix.texi

@@ -5286,7 +5286,9 @@ Once you have successfully booted the image on the USB stick, you should
 end up with a root prompt.  Several console TTYs are configured and can
 be used to run commands as root.  TTY2 shows this documentation,
 browsable using the Info reader commands (@pxref{Help,,, info, Info: An
-Introduction}).
+Introduction}).  The installation system runs the GPM mouse daemon,
+which allows you to select text with the left mouse button and to paste
+it with the middle button.
 
 To install the system, you would:
 

gnu/system/install.scm

@@ -306,6 +306,9 @@ You have been warned.  Thanks for being so brave.
           (console-font-service "tty5")
           (console-font-service "tty6")
 
+          ;; To facilitate copy/paste.
+          (gpm-service)
+
           ;; Since this is running on a USB stick with a unionfs as the root
           ;; file system, use an appropriate cache configuration.
           (nscd-service (nscd-configuration