libelf: Allow updating phdrs for any e_type.

libelf: Allow updating phdrs for any e_type.

elf[32|64]_updatenull would sanity check the e_type before allowing to
update the phdrs. This prevents creating an ET_REL file with phdrs. It
also prevents creating any vendor specific ELF file having phdrs. We
only check this when updating/writing out the file. But we would just
read such files. Don't prevent people from creating unexpected ELF files.
elflint will warn for such files.

While writing a new testcase for this another bug was found that
prevented updating a just created phdr because elf_getphdrnum would
sanity check the phdr offset in the file (which doesn't exist yet).
Fix that by only doing such a sanity check if the phdrs haven't been
read in or created yet.

This second bug should have been found by the existing elfshphehdr
test, but that test contained a typo checking elf_getphdrnum.
It tested that the called failed when there were no phdrs, but then
elf_getphdrnum should simply succeed and return zero.

https://bugzilla.redhat.com/show_bug.cgi?id=1352232

Signed-off-by: Mark Wielaard <mjw@redhat.com>