Unify temporary directory handling

Unify temporary directory handling

In archive_util.c, we have a private function named get_tempdir() which
is used by __archive_mktemp() to get the temporary directory if the
caller did not pass one.

In archive_read_disk_entry_from_file.c, we use the same logic with a
slight twist (don't trust the environment if setugid) to create a
temporary file for metadata.

Merge the two by renaming get_tempdir() to __archive_get_tempdir() and
unstaticizing it (with a prototype in archive_private.h).