Move Xen Kernel menuitem to Boot Options submenu. Prevent conflict with
upcoming Boot Environment menu. The Xen Kernel is dynamically appended
after the last menuitem in the Boot Options submenu. If xen_kernel is not
defined in loader.conf(5) the Xen Kernel menuitem displays "N/A" versus
"On"/"off". While here, fix-up the pkg-message (make capitalization of
"in" vs. "In" consistent; make use of trailing ":" consistent; move a
comment out of the way of file contents, etc.).

Differential Revision: https://reviews.freebsd.org/D2717
Reviewed by: allanjude, bapt
Approved by: bapt