Revamp tar_getcwd/normalize_filename stuff.

The changes are based on the discussion with Nathan.

* src/common.h (normalize_filename): Take two arguments. All
callers updated.
(tar_getcwd): Replaced with ..
(tar_getcdpath): New proto.
* src/misc.c (normalize_filename): Take two arguments.
(chdir_arg): Populate cwd along with creating the
structure.
(tar_getcwd): Removed.
(tar_getcdpath): New function.

* tests/incr09.at: New test case.
* tests/Makefile.am: Add new tests.
* tests/testsuite.at: Likewise.