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: NJason Riedy <ejr@cs.berkeley.edu>

fsck-cache.c

@@ -454,7 +454,7 @@ int main(int argc, char **argv)
 	if (standalone && check_full)
 		die("Only one of --standalone or --full can be used.");
 	if (standalone)
-		unsetenv("GIT_ALTERNATE_OBJECT_DIRECTORIES");
+		putenv("GIT_ALTERNATE_OBJECT_DIRECTORIES=");
 
 	fsck_head_link();
 	fsck_object_dir(get_object_directory());

receive-pack.c

@@ -269,7 +269,7 @@ int main(int argc, char **argv)
 
 	/* If we have a ".git" directory, chdir to it */
 	chdir(".git");
-	setenv("GIT_DIR", ".", 1);
+	putenv("GIT_DIR=.");
 
 	if (access("objects", X_OK) < 0 || access("refs/heads", X_OK) < 0)
 		die("%s doesn't appear to be a git directory", dir);

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;
 }