wrapper.c: add and use fopen_or_warn()

When fopen() returns NULL, it could be because the given path does not
exist, but it could also be some other errors and the caller has to
check. Add a wrapper so we don't have to repeat the same error check
everywhere.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ident.c

@@ -72,12 +72,10 @@ static int add_mailname_host(struct strbuf *buf)
 	FILE *mailname;
 	struct strbuf mailnamebuf = STRBUF_INIT;
 
-	mailname = fopen("/etc/mailname", "r");
-	if (!mailname) {
-		if (errno != ENOENT)
-			warning_errno("cannot open /etc/mailname");
+	mailname = fopen_or_warn("/etc/mailname", "r");
+	if (!mailname)
 		return -1;
-	}
+
 	if (strbuf_getline(&mailnamebuf, mailname) == EOF) {
 		if (ferror(mailname))
 			warning_errno("cannot read /etc/mailname");