xbps-install: don't treat EEXIST as an error when installing pkgs.

Thanks to pancake for the bug hunting.

NEWS

@@ -1,5 +1,8 @@
 xbps-0.20 (???):
 
+ * xbps-install: don't treat EEXIST as an error when installing
+   new packages; technically it's not an error.
+
  * libfetch: added a patch to fix select(2) timing out when fetching
    from some https servers.
 

bin/xbps-install/transaction.c

@@ -218,6 +218,7 @@ install_new_pkg(struct xbps_handle *xhp, const char *pkg, bool reinstall)
 	if ((rv = xbps_transaction_install_pkg(xhp, pkg, reinstall)) != 0) {
 		if (rv == EEXIST) {
 			printf("Package `%s' already installed.\n", pkg);
+			rv = 0;
 		} else if (rv == ENOENT) {
 			fprintf(stderr, "Unable to locate '%s' in "
 			    "repository pool.\n", pkg);