libdwfl: Add dwfl_module_addrsym_elf and dwfl_module_getsym_elf.

libdwfl: Add dwfl_module_addrsym_elf and dwfl_module_getsym_elf.

Introduce two new functions that also return the elf associated with a
symbol to make symbol section indexing work for non-special sections.
Simplify code by removing dwfl_file where appropriate and just track Elf
directly. Document limitations of shndx with existing dwfl_module_addrsym
and dwfl_module_getsym. Extend dwflsyms testcase to check some more symbol
and section (index) properties.

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