howto_names: unify declaration

Approved by: gibbs
Sponsored by: Citrix Systems R&D

boot/i386/efi/bootinfo.c:
boot/i386/libi386/bootinfo.c:
boot/ia64/common/bootinfo.c:
boot/powerpc/ofw/metadata.c:
boot/powerpc/ps3/metadata.c:
boot/sparc64/loader/metadata.c:
boot/uboot/common/metadata.c:
boot/userboot/userboot/bootinfo.c:
i386/xen/xen_machdep.c:

• Include sys/boot.h
• Remove custom definition of howto_names.

sys/boot.h:

• Define howto_names.

x86/xen/pv.c:

• Include sys/boot.h