RFR: 8293590: Some syntax checks performed by URL.openConnection() could be performed earlier, at URL construction

[Daniel Fuchs]

Many built-in URL Handlers perform additional syntax checking on the URL when `URLStreamHandler::openConnection` / connect is called. In some cases, some of these checks could be also performed earlier, when `URLStreamHandler::parseURL` is called.

This fix proposes to slightly modify the behavior of JDK built-in URL handlers to perform these checks early, and throw `MalformedURLException` earlier, at URL construction time, when that is possible, instead of delaying until `URL::openConnection` or `URLConnection::connect` are called.

Because java.net.URL constructors are already specified to throw `MalformedURLException` in such cases, no API documentation changes are necessary. 

A new JDK specific system property `-Djdk.net.url.delayParsing` or `-Djdk.net.url.delayParsing=true` can be specified on the command line to revert to the previous behavior. By default, the property is not set, and the new behavior is in place. The property can be specified on the java command line if unexpected regressions are observed.

The new property will be documented in release notes, and may be removed in a future release.

CSR: https://bugs.openjdk.org/browse/JDK-8294592

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Commit messages:
 - 8293590

Changes: https://git.openjdk.org/jdk/pull/10534/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10534&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293590
  Stats: 641 lines in 4 files changed: 602 ins; 3 del; 36 mod
  Patch: https://git.openjdk.org/jdk/pull/10534.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10534/head:pull/10534

PR: https://git.openjdk.org/jdk/pull/10534