vnet.9: Rename vnet.9/vimage.9 to VNET.9/VIMAGE.9

Fix crossreferences to VNET(9) by correcting it's capitalization.

MFC after: 3 days
Reviewed by: enji, ziaee
Closes: https://github.com/freebsd/freebsd-src/pull/1848

(cherry picked from commit 3049b2f5115b3b4aed202274e866f360fa2560d1)
(cherry picked from commit 8f7a4d7949d83c13a7451cba7626950d182dfc5d)
(cherry picked from commit 08d41d8803d9dd49b709518e8f2ce292b59e9828)