Move KHELP_DECLARE_MOD_UMA later in the boot

Both KHELP_DECLARE_MOD_UMA and the kernel linker SYSINIT to find
in-kernel modules run at SI_SUB_KLD, SI_ORDER_ANY. As the former
depends on the latter running first move it later in the boot,
to the new SI_SUB_KHELP. This ensures KHELP_DECLARE_MOD_UMA
module SYSINIT functions will be after the kernel linker.

Previously we may have received a panic similar to the following if
the order was incorrect:

panic: module_register_init: module named ertt not found

Reported by: bob prohaska <fbsd AT www.zefox.net>
Discussed with: imp, jhb
Sponsored by: The FreeBSD Foundation