Improve tar_getcwd

Improve tar_getcwd

* src/common.h (tar_getcwd): Return pointer is const.
* src/misc.c (wd) <cwd>: New member.
(chdir_arg): Initialize cwd.
(tar_getcwd): Use cwd member to cache the result.  Take into
account absolute pathnames,
(normalize_filename): Don't free the value
returned from tar_getcwd.
* src/names.c (name_next_elt): Remove leftover call chdir().
* tests/Makefile.am: Add new tests.
* tests/testsuite.at: Likewise.

* tests/incr08.at: New testcase.
* tests/remfiles04.at: New testcase.
* tests/remfiles05.at: New testcase.
* tests/remfiles06.at: New testcase.
* tests/remfiles07.at: New testcase.