stand: some misc cleanup

[VexedUXR]

Some cleanup commits, as promised in #1098 :)

This series, along with some minor changes, does two main things:

• Move some common macros to headers.
• Use headers instead of doing things ourselves in the various elf64_freebsd.c files.

A note about moving bi_load into loader_efi.h: Almost every file that includes loader_efi.h proceeds to do some form of

extern int bi_load(char *, vm_offset_t *, vm_offset_t *, bool);

Since this is the efi version of bi_load, I moved it info loader_efi.h. However, the kboot files don't always include loader_efi.h, so it looks like we won't be able to remove the declaration there. I'm open to suggestions about where else to put bi_load (or any of the other things I moved), but I don't see many options.

[bsdimp]

I think we may need to snag the 'no brainer yes' bits and iterate. So I'll look at doing that.