release: Set loader_menu_multi_user_prompt to "Installer" for installer

This makes it more obvious that the media being booted is an installer
rather than an installed system, which is otherwise hard to distinguish.
It also provides a more user-friendly, and more accurate, prompt.

Reviewed by: gjb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36419

(cherry picked from commit 2a90c47af158625b3e28390801639c21d6d0ed2a)