This allows the "Multi user" in "[B]oot Multi user" to be substituted
with another string, for example with "Installer" in installer media.
Note that this is lua-only at the moment, since loader.4th's menu.rc
defines the alternate name as Boot [M]ulti User, unlike lualoader which
leaves it as [B]oot Multi user. Ideally loader.4th would adopt the newer
and simpler lualoader behaviour and then it could gain support for this
option, but loader.4th is on the way out and isn't used by any official
installer media so this is not a significant concern.
Reviewed by: kevans, rpokala
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36418
9b17aa2740