libelf: gelf_newehdr and gelf_newehdr should return void *.

libelf: gelf_newehdr and gelf_newehdr should return void *.

unsigned long int is not always capable to have pointer in some cases
(LLP64, for example). Return a void pointer instead. Other libelf
implementations will also make this change (or already have).
Also update the documentation to state what is created and that NULL
is returned on error (don't document that the returned value is a
pointer to the actual header created).

Signed-off-by: Akihiko Odaki <akihiko.odaki.4i@stu.hosei.ac.jp>
Signed-off-by: Mark Wielaard <mjw@redhat.com>