replication: relax rules box.cfg.*_name

We decided to allow underscore in names to provide easier support
for many existent installations.

Closes #9148

NO_CHANGELOG=no updates to the feature description
NO_DOC=update existent doc tickets 3466, 3467, 3468

src/box/node_name.c

@@ -15,7 +15,7 @@ node_name_is_valid_n(const char *name, size_t len)
 	const char *end = name + len;
 	while (name < end) {
 		char c = *(name++);
-		if (!isalnum(c) && c != '-')
+		if (!isalnum(c) && c != '-' && c != '_')
 			return false;
 		if (tolower(c) != c)
 			return false;

test/unit/node_name.c

@@ -15,7 +15,7 @@ test_node_name_is_valid(void)
 		"1",
 		"1abc",
 		"*",
-		"a_b",
+		"a+b",
 		"aBcD",
 		"a~b",
 		"{ab}",
@@ -28,7 +28,7 @@ test_node_name_is_valid(void)
 	}
 	const char *good_names[] = {
 		"a",
-		"a-b-c",
+		"a_b-c",
 		"abc",
 		"a1b2c3-d4-e5-",
 	};