libelf: treat phdr and shdr similarly

libelf: treat phdr and shdr similarly

Just like elf_shdr_count(), elf_phdr_count() better bounds checks the
value.

Add table entry size checks to elf_init().

Also both program and section headers are optional, and hence their
checking better is done conditionally only when any such headers are
present.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>