RFR: 8190312: javadoc -link doesn't work with http: -> https: URL redirects

[Jonathan Gibbons]

Please review a fix for javadoc such that it will follow redirected 
URLs, including http: to https:, for URLs given on the command line with 
-link.

An unconditional warning is given if it is found that a URL is redirected.

The change is in Extern.java, with the primary change being the addition 
of a new method `open(URL)` that allows redirection and generates the 
warning message.  The other changes are cosmetic cleanup, partly to 
aggregate some URL handling methods together at the end of the file, and 
partly to follow IDE suggestions.

The bigger part of the work is a new test, with two test cases.

The first test case is a "real world" test case that uses 
docs.oracle.com if it is available.  Running this test case may require 
proxies to be set in order to work as intended. The test checks if the 
site can be accessed, and skips the test case if not.

The second case sets up two transient webservers, with an HttpServer 
that redirects all requests to an HttpsServer. The test case uses SSL 
credentials used in similar tests in the main test/jdk test suite. This 
test case is always enabled. The test case verifies that the warning 
message is generated as expected and that the generated files do -not- 
use the redirected URLs. This is to avoid baking in an assumption that 
all the files will be redirected. In other words, the redirection is 
only used for reading the element-list or package-list files.

JBS: https://bugs.openjdk.java.net/browse/JDK-8190312
Webrev: http://cr.openjdk.java.net/~jjg/8190312/webrev.00/

-- Jon