network: update comment and log message

After 4b30f2e135ee84041bb597edca7225858f4ef4fb, reading stable_secret
sysctl property fails with -ENOMEM, instead of -EIO.
This is due to read_full_virtual_file() uses read() as the backend while
read_one_line_file() uses fgetc(). And each functions return different
error on fails.

Anyway, the failure is harmless here. So, the log message and comment is
updated.

Closes one of the issues in #19410.

src/core/service.c

@@ -3480,7 +3480,7 @@ static void service_sigchld_event(Unit *u, pid_t pid, int code, int status) {
                                 if (control_pid_good(s) <= 0)
                                         service_enter_stop(s, f);
 
-                                /* Otherwise need to wait untill the operation is done. */
+                                /* Otherwise need to wait until the operation is done. */
                                 break;
 
                         case SERVICE_STOP:

src/network/networkd-conf.c

@@ -18,6 +18,8 @@ int manager_parse_config_file(Manager *m) {
                         PKGSYSCONFDIR "/networkd.conf",
                         CONF_PATHS_NULSTR("systemd/networkd.conf.d"),
                         "Network\0"
+                        "DHCPv4\0"
+                        "DHCPv6\0"
                         "DHCP\0",
                         config_item_perf_lookup, networkd_gperf_lookup,
                         CONFIG_PARSE_WARN,

src/network/networkd-dhcp6.c

@@ -888,7 +888,7 @@ static int dhcp6_pd_prefix_add(Link *link, const union in_addr_union *prefix, ui
                       "DHCP6: received PD Prefix %s%s",
                       strna(buf),
                       prefixlen > 64 ? " with prefix length > 64, ignoring." :
-                      prefixlen < 48 ? " with prefix lenght < 48, looks unusual.": "");
+                      prefixlen < 48 ? " with prefix length < 48, looks unusual.": "");
 
         /* Store PD prefix even if prefixlen > 64, not to make logged at warning level so frequently. */
         r = set_ensure_put(&link->dhcp6_pd_prefixes, &in_addr_prefix_hash_ops_free, p);

src/network/networkd-link.c

@@ -1327,8 +1327,9 @@ static int link_configure_addrgen_mode(Link *link) {
                 r = sysctl_read_ip_property(AF_INET6, link->ifname, "stable_secret", NULL);
                 if (r < 0) {
                         /* The file may not exist. And even if it exists, when stable_secret is unset,
-                         * reading the file fails with EIO. */
-                        log_link_debug_errno(link, r, "Failed to read sysctl property stable_secret: %m");
+                         * reading the file fails with ENOMEM when read_full_virtual_file(), which uses
+                         * read() as the backend, and EIO when read_one_line_file() which uses fgetc(). */
+                        log_link_debug_errno(link, r, "Failed to read sysctl property stable_secret, ignoring: %m");
 
                         ipv6ll_mode = IN6_ADDR_GEN_MODE_EUI64;
                 } else