RFR: JDK-6251738: Want a top-level summary page that itemizes all spec documents referenced from javadocs (OEM spec)

[Jonathan Gibbons]

This introduces support for a new `@spec` tag that can be used as either an inline tag or as a block tag. It is used to identify references to external specifications, in such a way that the references can be collected together on a new summary page, called "Other Specifications", available from either the static INDEX page or the interactive search box.

As an inline tag, the format is `{@spec url label}`, which is roughly translated to `<a href="url">label</a>` in the generated docs.

As a block tag, the format is simply

    @spec url label

which is handled in a manner analogous to

    @see <a href="url">label</a>

The tag is notable for being the first standard/supported tag that can be used as either an inline or block tag. (We have had support for bimodal non-standard/custom tags for a while, such as `{@jls}` and `{@jvms}`.) To support bimodal standard tags, some changes to `DocCommentParser` are incorporated here.

This change is only the _support_ for the new tag;  it does _not_ include any changes to API docs to _use_ the new tag.

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Commit messages:
 - fix TestSpecTag.testEncodedURI
 - fix tests
 - remove support to workaround legacy @spec tag
 - Merge remote-tracking branch 'upstream/master' into new-spec-tag
 - fix trailing whitespace in test
 - temporarily allow existing legacy usage `@spec JPMS` `@spec jsr-51`
 - JDK-6251738: Want a top-level summary page that itemizes all spec documents referenced from javadocs (OEM spec)

Changes: https://git.openjdk.java.net/jdk/pull/790/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=790&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-6251738
  Stats: 1376 lines in 42 files changed: 1345 ins; 12 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/790.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/790/head:pull/790

PR: https://git.openjdk.java.net/jdk/pull/790