Fix setting of invocation-directory with --chdir and relative argv[0]

Fix setting of invocation-directory with --chdir and relative argv[0]

* src/emacs.c (original_pwd): New char.
(main): If using --chdir, store original_pwd.
(init_cmdargs): When setting Vinvocation_directory based on a
relative argv[0], use original_pwd if set.

Fixes: debbugs:15768