installer: Add a help page.

installer: Add a help page.

* gnu/installer/newt/help.scm: New file.
* gnu/local.mk (INSTALLER_MODULES): Add it.
* po/guix/POTFILES.in: Add it.
* gnu/installer/record.scm (<installer>): Add 'help-menu' and 'help-page'
 fields,
 (installer-help-menu, installer-help-page): new exported procedures.
 * gnu/installer/newt.scm (init): Set the help line,
 (help-menu, help-page): new procedures used ...
 (newt-installer): ... here.
 * gnu/installer/newt/keymap.scm (run-layout-page): Add a context argument to
 differenciate the help context from the main one,
 (run-keymap-page): add a context argument and pass it to run-layout-page.
 * gnu/installer.scm (compute-keymap-step): Add a context argument and pass it
 to 'installer-keymap-page',
 (installer-steps): set the help menu and pass the appropriate context to
 compute-keymap-step calls,
 (guile-newt): update to revision 2.