libdwfl: Move dwz alt multi file searching to find_debuginfo callback.

libdwfl: Move dwz alt multi file searching to find_debuginfo callback.

Don't hard code the Dwarf dwz alt multi file search but allow the user
to override it through the standard Dwfl_Callbacks. Also move ownership
completely to the user of dwarf_setalt by removing free_alt from Dwarf
and adding alt, fd and elf fields to Dwfl_Module. Add a relative .dwz
file test case.

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