elf_newdata: Make sure to set d_type to ELF_T_BYTE and d_align to one

elf_newdata: Make sure to set d_type to ELF_T_BYTE and d_align to one

If d_align isn't explicitly set in elf_newdata it will default to
zero. This confuses elf_update when more than one Elf_Data has been
added to a (new) Elf_Section. Only the last will appear to be added.
ELF_T_BYTE happens to be zero and so d_type did appear to be setup
correctly. Also set it explicitly to show the defaults.

The elf_newdata test happened to pass because it set d_align to 1
explicitly. But would fail if it wouldn't set it. Add some extra
checks.

Also update the elf_newdata man page with the new defaults.

	* libelf/elf_newdata.c (elf_newdata): Explicitly set d_type
	and d_align.
	* tests/newdata.c (add_section_data): Check d_buf, d_size,
	d_type, d_align and d_version default values.
	* doc/elf_newdata.3: Document d_align defaults to one.

Signed-off-by: Mark Wielaard <mark@klomp.org>