libdw: dwarf_begin_elf should use elf_getshdrstrndx to get section names.

libdw: dwarf_begin_elf should use elf_getshdrstrndx to get section names.

dwarf_begin_elf used the Ehdr e_shstrndx to get the shdr string table
section. This does not work for ELF files with more than SHN_LORESERVE
sections. Use elf_getshdrstrndx, and don't pass around the ehdr.

Add a simple testcase that fails before the patch because dwarf_begin
return an error.

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