Add a dummy vendor menu entry on the main welcome menu. Vendors can override
this in their local.lua file to create whatever sub-menu they need for their
products.
Also fix the adding a 'welcome' entry as well based on a suggestion from Kyle.
Silly option menu code also from Kyle. They seem to work for me, but any
transcription error is likely mine.
Reviewed by: kevans@ (the vendor stuff)
Distribution will be done after all of the lualoader manpages are created.
Reviewed by: rpokala
Differential Revision: https://reviews.freebsd.org/D14480
