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