Init $g_display_errors with dev settings on localhost

When the server name is localhost/127.0.0.1, we automatically set
$g_display_error to the recommended development values, i.e.

 - E_USER_ERROR   => DISPLAY_ERROR_HALT
 - E_WARNING      => DISPLAY_ERROR_HALT
 - E_ALL          => DISPLAY_ERROR_INLINE

Issue #17501

config_defaults_inc.php

@@ -4084,7 +4084,8 @@
  * The default settings are recommended for use in Production, and will only
  * display MantisBT fatal errors, suppressing output of all other error types.
  *
- * Recommended config_inc.php settings for developers:
+ * Recommended config_inc.php settings for developers (these are automatically
+ * set if the server is localhost):
  * $g_display_errors = array(
  *     E_USER_ERROR   => DISPLAY_ERROR_HALT,
  *     E_WARNING      => DISPLAY_ERROR_HALT,
@@ -4101,6 +4102,15 @@
 	E_USER_ERROR   => DISPLAY_ERROR_HALT,
 );
 
+# Add developers defaults when server is localhost
+# Note: intentionally not using SERVER_ADDR as it's not guaranteed to exist
+if( strcasecmp( $_SERVER['SERVER_NAME'], 'localhost' ) == 0
+ || $_SERVER['SERVER_NAME'] == '127.0.0.1'
+) {
+	$g_display_errors[E_WARNING] = DISPLAY_ERROR_HALT;
+	$g_display_errors[E_ALL]     = DISPLAY_ERROR_INLINE;
+}
+
 /**
  * Detailed error messages
  * Shows a list of variables and their values when an error is triggered.

docbook/Admin_Guide/en-US/Configuration.xml

@@ -3527,6 +3527,13 @@ $g_display_errors = array(
 );
 </programlisting>
 					</para>
+					<note>
+						<para>The system automatically sets
+							<emphasis>$g_display_errors</emphasis>
+							to the above recommended development values when
+							the server's name is <emphasis>localhost</emphasis>.
+						</para>
+					</note>
 					<para>Less intrusive settings, recommended for testing purposes:
 <programlisting>
 $g_display_errors = array(