system: Add "installer" sub-command.

* guix/scripts/system/installer.scm: New file.
* Makefile.am (MODULES)[ENABLE_INSTALLER]: Register it.
(MODULES_NOT_COMPILED)[!ENABLE_INSTALLER]: Likewise.
* guix/scripts/system.scm (show-help): Add help for "installer" sub-command.
(actions): Register "installer".
(guix-system): Invoke `guix-system-installer' sub-command.
* doc/guix.texi (Invoking guix system): Document it.
* gnu/installer.scm (run-installer): Remove "./pre-inst env".

Change-Id: I5a05b941c060682c17d45d871df3cf34e3f8643a

doc/guix.texi

@@ -43258,6 +43258,23 @@ This command also installs bootloader on the targets specified in
 @file{my-os-config}, unless the @option{--no-bootloader} option was
 passed.
 
+@item installer
+Run the installer.  Usually the installer is built as an @file{iso}
+image, copied to a USB Stick or DVD, and booted from (@ref{USB Stick and
+DVD Installation}).  If your machine already runs Guix and you still
+want to run the installer, e.g., for testing purposes, you can skip the
+step of creating an @file{iso} and run for instance:
+
+@example
+guix system installer --dry-run
+@end example
+
+@quotation Note
+If you do not use @option{--dry-run} then you need to run as root.  Be
+@emph{very careful} when running the installer as root, it can cause
+data loss or render your system unbootable!
+@end quotation
+
 @item vm
 @cindex virtual machine
 @cindex VM