Apparently sending the tar output to stdout rather than a file causes tar to

pad with trailing NULs, which slightly upsets old versions of gzip (1.2.4,
not 1.3.x), which upsets some of our correspondents.
Use -f to send it to a file instead. (It's not transparently clear what
happens when you mix -f and -C; I've tested this with ixion's tar, 1.13.25.)

Also add option -o to generate POSIX tar files, as for halibut/Makefile;
apparently this shuts up a `trailing garbage' error from WinZip.

git-svn-id: svn://svn.tartarus.org/sgt/putty@4429 cda61777-01e9-0310-a592-d414129be87e