iconvconfig: Fix multiple issues

iconvconfig: Fix multiple issues

It was noticed on big-endian systems that msgfmt would fail with the
following error:

msgfmt: gconv_builtin.c:70: __gconv_get_builtin_trans: Assertion `cnt < sizeof (map) / sizeof (map[0])' failed.
Aborted (core dumped)

This is only seen on installed systems because it was due to a
corrupted gconv-modules.cache.  iconvconfig had the following issues
(it was specifically freeing fulldir that caused this issue, but other
cleanups are also needed) that this patch fixes.

- Add prefix only if dir starts with '/'
- Use asprintf instead of mempcpy so that the directory string is NULL
  terminated
- Make a copy of the directory reference in new_module so that fulldir
  can be freed within the same scope in handle_dir.

Reviewed-by: Florian Weimer <fweimer@redhat.com>