Validate External Links/HTTP codes

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ValExtLinks prints the HTTP response code for each link (except EI and IW links) in parentheses after the OK/NG/RD status of the link. Based on this Wikipedia list, following are the codes understood by ValExtLinks. When no HTTP response could be retrieved by the Unix tool 'curl' which queries each link, you'll see "(000-xx)", where the one- or two-digit number after the dash is the 'curl' error code.

2xx: Success

This class of status codes indicates the action requested by the client was received, understood, accepted, and processed successfully.

200 OK
Standard response for successful HTTP requests.

3xx: Redirection

This class of status code indicates the client must take additional action to complete the request. Many of these status codes are used in URL redirection.

301 Moved Permanently
This and all future requests should be directed to the given URI.
302 Found
This is an example of industry practice contradicting the standard. The HTTP/1.0 specification required the client to perform a temporary redirect (the original describing phrase was "Moved Temporarily"), but popular browsers implemented 302 with the functionality of a 303 See Other. Therefore, HTTP/1.1 added status codes 303 and 307 to distinguish between the two behaviors. However, some Web applications and frameworks use the 302 status code as if it were the 303.
303 See Other
The response to the request can be found under another URI using a GET method.
307 Temporary Redirect
The request should be repeated with another URI; however, future requests should still use the original URI.
308 Permanent Redirect
The request and all future requests should be repeated using another URI.

4xx: Client Errors

This class of status codes is intended for situations in which the client seems to have erred.

400 Bad Request
The server cannot or will not process the request due to an apparent client error (e.g., malformed request syntax).
401 Unauthorized
Similar to 403 Forbidden, but specifically for use when authentication is required and has failed or has not yet been provided.
403 Forbidden
The request was valid, but the server is refusing action. The user might not have the necessary permissions for a resource.
404 Not Found
The requested resource could not be found but may be available in the future. Subsequent requests by the client are permissible.
405 Method Not Allowed
A request method is not supported for the requested resource; for example, a GET request on a form that requires data to be presented via POST, or a PUT request on a read-only resource.
406 Not Acceptable
The requested resource is capable of generating only content not acceptable according to the Accept headers sent in the request.
410 Gone
Indicates that the resource requested is no longer available and will not be available again. This should be used when a resource has been intentionally removed and the resource should be purged. Upon receiving a 410 status code, the client should not request the resource in the future.
418 I'm a teapot
This is a joke response code used by some sites. Probably equivalent to a 200.

5xx: Server Errors

These codes mean that the server failed to fulfill an apparently valid request.

500 Internal Server Error
A generic error message, given when an unexpected condition was encountered and no more specific message is suitable.
501 Not Implemented
The server either does not recognize the request method, or it lacks the ability to fulfill the request. Usually this implies future availability (e.g., a new feature of a web-service API).
502 Bad Gateway
The server was acting as a gateway or proxy and received an invalid response from the upstream server.
503 Service Unavailable
The server is currently unavailable (because it is overloaded or down for maintenance). Generally, this is a temporary state.
530 Site is frozen
Some Internet companies use this to indicate that a site is down, but usage is non-standard, so the site will have to be tried again later to see if the outage is due to a frozen account or something else.