Fix normalize_filename.

The function did not take into account eventual -C options, which
in particular led to various problems when using -C and --remove-files
together.

* src/common.h (namebuf_add_dir,namebuf_finish)
(tar_getcwd): New prototypes.
* src/misc.c (namebuf_add_dir,namebuf_finish)
(tar_getcwd): New functions.
(normalize_filename): Use tar_getcwd.