RFR: 8262442 (windows) use all proxy configuration sources when java.net.useSystemProxies=true

[Daniel Jeliński]

With this patch DefaultProxySelector first attempts to use proxy config autodetection (http://wpad/wpad.dat) when that is configured and available.
If proxy config autodetection is unavailable, selector tries to use configured proxy script (again, if configured and available)
If both the above options fail, selector uses the configured proxy.

Verified on Windows 10 that:
- when `fAutoDetect` is true, http://wpad/wpad.dat refers to an existing file, the file has correct syntax and returns a proxy, that proxy is used
- when `fAutoDetect` is true, but the autoconfig file is not available / unusable for any reason, selector fails over to the next configured method
- when `lpszAutoConfigUrl` is set and usable, the proxy returned is used
- when `lpszAutoConfigUrl` is not set or unusable, selector fails over to next method
- when `lpszProxy` is configured, that proxy is used
- otherwise selector uses direct connection

The proxy configuration scripts are cached on system level, so testing (alternating between good and broken autoconfig script) may require waiting for the caches to invalidate.

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Commit messages:
 - Use all proxy configs

Changes: https://git.openjdk.java.net/jdk/pull/5995/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5995&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262442
  Stats: 14 lines in 1 file changed: 7 ins; 5 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5995.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5995/head:pull/5995

PR: https://git.openjdk.java.net/jdk/pull/5995