* tar.c: New file; built-in tar using zlib.

* tar.c: New file; built-in tar using zlib.
* tar.h: New file; header for same.
* ctar.c: New file; test program for same.
* Makefile.in: add built-in tar, remove tar.exe and gzip.exe
* path.c (pathcat): convert slashes and canonicalize, instead of aborting
* setup.c (tarx): add support for built-in tar
(main): accept unix-style slashes, check root dir for drive letters,
warn about installing in /, make sure cwd is empty for internet installs,
defer mounts to end of install, support built-in tar.