libdw: Cache ELF directory early. Explicitly set it in dwfl.

libdw: Cache ELF directory early. Explicitly set it in dwfl.

The logic that finds alt files and dwo files relies on having an open
file descriptor. But after all needed ELF data has been read the
underlying Elf file descriptor can be closed. libdwfl in particular
closes file descriptor fairly aggressively. So capture the directory
early on. And make dwfl set it if it has recorded it. Which it will
do now before closing a file descriptor for the main Dwfl_Module file.

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