installer: Choosing a locale opens the translated manual on tty2.

installer: Choosing a locale opens the translated manual on tty2.

Suggested by Florian Pelz.

* gnu/system/install.scm (%installation-node-names): New variable.
(log-to-info): Expect the chosen locale as an argument.  Compute the
language, Info file name, and node name.  Install the locale.
(documentation-shepherd-service): Add 'locale' parameter to the 'start'
action and honor it.  Set GUIX_LOCPATH and TERM as environment variables
for the process.
* gnu/installer.scm (apply-locale): Use (gnu services herd).  Call
'stop-service' and 'start-service' with the chosen locale.