bsd-user: Refactor load_elf_sections and is_target_elf_binary

Factor out load_elf_sections and is_target_elf_binary out of
load_elf_interp.

Signed-off-by: Mikaël Urankar <mikael.urankar@gmail.com>
Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Kyle Evans <kevans@FreeBSD.org>