Use relative addressing in deferred unlinks.

* src/common.h (tar_dirname): New function.
* src/misc.c (normalize_filename_x): Make extern.
(tar_dirname): New function.
(tar_getcwd): Take into account absoulte pathnames.
* src/unlink.c (deferred_unlink) <dir_idx>: New member; keeps the
value of chdir_current at the moment of structure allocation.
(flush_deferred_unlinks): Use chdir_do and relative addressing.
(queue_deferred_unlink): Initialize dir_idx.
* tests/Makefile.am: Add new tests.
* tests/testsuite.at: Add new tests.
* tests/remfiles06.at: Fix description.
* tests/remfiles07.at: Fix description.
* tests/remfiles08.at: New test case.