Separate /boot/kernel/*.symbols into its own distribution
set, kernel_symbols.txz, which is selected by default.

Provide a manifest description for kernel_symbols.txz.

Fix installing LOCAL_DISTRIBUTIONS by properly setting the
BSDINSTALL_FETCHDEST and BSDINSTALL_DISTDIR environment
variables, enforcing the local MANIFEST is used.

When generating DISTMENU (used to populate the available
distribution sets for selection), do not exclude all of
the available base* and kernel* distributions, which allows
the base-dbg.txz and kernel.*.txz to be listed.

Sponsored by: The FreeBSD Foundation