Updated in accordance with feedback from @narfbg

- Removed commented lists of constants from the three reference conventions,
  replacing each with the URLs at which more information can be found.
- Renamed a few constants to more closely reflect CodeIgniter conventions.
- Modified a couple of lines which were in violation of the CI Style Guide.

Signed-off-by: Daniel Hunsaker <danhunsaker@gmail.com>
diff --git a/system/core/Common.php b/system/core/Common.php
index 479f0da..e11668d 100644
--- a/system/core/Common.php
+++ b/system/core/Common.php
@@ -177,7 +177,7 @@
 			// self-referencing loop with the Exceptions class
 			set_status_header(503);
 			echo 'Unable to locate the specified class: '.$class.'.php';
-			exit(EXIT_UNK_CLASS);
+			exit(EXIT_UNKNOWN_CLASS);
 		}
 
 		// Keep track of what we just loaded
@@ -377,13 +377,13 @@
 			$exit_status = $status_code + EXIT__AUTO_MIN;
 			if ($exit_status > EXIT__AUTO_MAX)
 			{
-				$exit_status = EXIT_FAILURE;
+				$exit_status = EXIT_ERROR;
 			}
 			$status_code = 500;
 		}
 		else
 		{
-			$exit_status = EXIT_FAILURE;
+			$exit_status = EXIT_ERROR;
 		}
 		
 		$_error =& load_class('Exceptions', 'core');
@@ -411,7 +411,7 @@
 	{
 		$_error =& load_class('Exceptions', 'core');
 		$_error->show_404($page, $log_error);
-		exit(EXIT_UNK_FILE);
+		exit(EXIT_UNKNOWN_FILE);
 	}
 }
 
@@ -531,13 +531,16 @@
 
 		$server_protocol = isset($_SERVER['SERVER_PROTOCOL']) ? $_SERVER['SERVER_PROTOCOL'] : FALSE;
 
-		if (strpos(php_sapi_name(), 'cgi') === 0)
+		if ( ! headers_sent())
 		{
-			if (!headers_sent()) header('Status: '.$code.' '.$text, TRUE);
-		}
-		else
-		{
-			if (!headers_sent()) header(($server_protocol ? $server_protocol : 'HTTP/1.1').' '.$code.' '.$text, TRUE, $code);
+			if (strpos(php_sapi_name(), 'cgi') === 0)
+			{
+				header('Status: '.$code.' '.$text, TRUE);
+			}
+			else
+			{
+				header(($server_protocol ? $server_protocol : 'HTTP/1.1').' '.$code.' '.$text, TRUE, $code);
+			}
 		}
 	}
 }
diff --git a/system/core/Exceptions.php b/system/core/Exceptions.php
index 423387f..9c68d06 100644
--- a/system/core/Exceptions.php
+++ b/system/core/Exceptions.php
@@ -117,7 +117,7 @@
 		}
 
 		echo $this->show_error($heading, $message, 'error_404', 404);
-		exit(EXIT_UNK_FILE);
+		exit(EXIT_UNKNOWN_FILE);
 	}
 
 	// --------------------------------------------------------------------
diff --git a/system/helpers/download_helper.php b/system/helpers/download_helper.php
index 25863ea..bd32965 100644
--- a/system/helpers/download_helper.php
+++ b/system/helpers/download_helper.php
@@ -142,7 +142,7 @@
 		if ($data !== NULL)
 		{
 			echo $data;
-			exit(EXIT_SUCCESS);
+			exit;
 		}
 
 		// Flush 1MB chunks of data
@@ -152,7 +152,7 @@
 		}
 
 		fclose($fp);
-		exit(EXIT_SUCCESS);
+		exit;
 	}
 }
 
diff --git a/system/libraries/Driver.php b/system/libraries/Driver.php
index 9a56013..1bc365c 100644
--- a/system/libraries/Driver.php
+++ b/system/libraries/Driver.php
@@ -290,7 +290,7 @@
 
 		$trace = debug_backtrace();
 		_exception_handler(E_ERROR, "No such method '{$method}'", $trace[1]['file'], $trace[1]['line']);
-		exit(EXIT_UNK_MEMBER);
+		exit(EXIT_UNKNOWN_METHOD);
 	}
 
 	// --------------------------------------------------------------------
diff --git a/system/libraries/Xmlrpcs.php b/system/libraries/Xmlrpcs.php
index e150c13..a6048cb 100644
--- a/system/libraries/Xmlrpcs.php
+++ b/system/libraries/Xmlrpcs.php
@@ -171,7 +171,7 @@
 		header('Content-Type: text/xml');
 		header('Content-Length: '.strlen($payload));
 		echo $payload;
-		exit(EXIT_SUCCESS);
+		exit;
 	}
 
 	// --------------------------------------------------------------------