share/man/man[49]: Set PACKAGE=kernel-man

When MANSPLITPKG is disabled (which is now the default), the kernel
manpages in sections 4 and 9 are installed in the -kernel package
instead of the -kernel-man package, because bsd.man.mk doesn't add
the -man suffix.

Since we already have handling for kernel-man in generate-ucl.lua,
set PACKAGE=kernel-man to make sure they're always in the right
package.

To avoid creating -kernel-man-man when WITH_MANSPLITPKG is enabled,
add some logic to bsd.man.mk (suggested by kevans) to avoid adding
the -man suffix if the package already has a -man suffix, and while
here, ensure ${__group}PACKAGE is always defined.

MFC after: 3 seconds
Reviewed by: des, bapt
Differential Revision: https://reviews.freebsd.org/D52525

(cherry picked from commit 6d83ae143b1f598b5f8868508f6e8a7d3440f568)