Replace unsetenv() and setenv() with older putenv().

Solaris 8 doesn't have the newer unsetenv() and setenv()
functions, so replace them with putenv().  The one use of
unsetenv() in fsck-cache.c now sets GIT_ALTERNATE_OBJECT_
DIRECTORIES to the empty string.  Every place that var
is used, NULLs are also replaced with empty strings, so
it's ok.

Signed-off-by: Jason Riedy <ejr@cs.berkeley.edu>

upload-pack.c

@@ -179,7 +179,7 @@ int main(int argc, char **argv)
 	chdir(".git");
 	if (access("objects", X_OK) || access("refs", X_OK))
 		die("git-upload-pack: %s doesn't seem to be a git archive", dir);
-	setenv("GIT_DIR", ".", 1);
+	putenv("GIT_DIR=.");
 	upload_pack();
 	return 0;
 }