t5003: check if unzip supports symlinks

t5003: check if unzip supports symlinks

Only add a symlink to the repository if both the filesystem and
unzip support symlinks.  To check the latter, add a ZIP file
containing a symlink, created like this with InfoZIP zip 3.0:

	$ echo sample text >textfile
	$ ln -s textfile symlink
	$ zip -y infozip-symlinks.zip textfile symlink

If we can extract it successfully, we add a symlink to the test
repository for git archive --format=zip, or otherwise skip that
step.  Users can see the skipped test and perhaps run it again
with a different unzip version.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>