Rewrite update algorithm.

Rewrite update algorithm.

* src/common.h (namebuf_t): New typedef.
(namebuf_create, namebuf_free)
(namebuf_name): New prototypes.
(remname): New prototype.
* src/misc.c (struct namebuf): New structure.
(namebuf_create, namebuf_free)
(namebuf_name): New functions.
* src/create.c (dup_dir0): Remove is_avoided_name
checks. This is taken care of in update_archive.
* src/incremen.c (scan_directory): Use namebuf
to produce full file names.
* src/names.c (nametail): Remove extra level of
indirection. All uses updated.
(avoided_name_table, add_avoided_name)
(is_avoided_name): Remove.
* src/update.c (update_archive): Change algorithm.
Instead of adding unmodified files to the avoided_name
table, create namelist so that it contains only
modified files.

* tests/Makefile.am: Add update01.at, update02.at
* tests/testsuite.at: Likewise.
* tests/update.at (AT_KEYWORDS): Add update00.