Limit username length and pattern in system yang model

Deviation in ietf-system.yang
'replace' username type from general string to a string
with length and pattern restrictions inline with the
BusyBox 'adduser' tool.

Note, chk_sr_user_update is more restrictive, which should
be handled.

src/confd/yang/infix-system@2023-07-04.yang

@@ -14,9 +14,10 @@ module infix-system {
 
   revision 2023-07-04 {
     description "Updating/adding deviation specifications:
-                 - timezone-utc-offset (updated)
-                 - radius (added)
-                 - dns-resolver port (added)";
+                 - timezone-utc-offset (updated path for 'not-supported')
+                 - radius ('not-supported')
+                 - dns-resolver port ('not-supported')
+                 - authentication username (limit length and pattern)";
     reference "internal";
   }
 
@@ -25,6 +26,13 @@ module infix-system {
     reference "internal";
   }
 
+  typedef username {
+    type string {
+      length "1..256";
+      pattern "[_a-zA-Z0-9][-._a-zA-Z0-9]*$?";
+    }
+  }
+
   augment "/sys:system" {
     description "Augments to ietf-system not found in any other model.";
     leaf motd {
@@ -44,4 +52,10 @@ module infix-system {
   deviation "/sys:system/sys:dns-resolver/sys:server/sys:transport/sys:udp-and-tcp/sys:udp-and-tcp/sys:port" {
     deviate not-supported;
   }
+
+  deviation "/sys:system/sys:authentication/sys:user/sys:name" {
+    deviate replace {
+      type infix-sys:username;
+    }
+  }
 }