release-version.sh: tag pom-fiji releases normally

Some multi-module projects use a "pom-foo" style aggregator, which we
want to tag as simply "foo" when doing releases. Fiji used to be like
this, but now the Fiji repositories are all split up. So for pom-fiji,
let's use the regular prefix of "pom-fiji" when tagging.
check-release.pl: rewrite to be more awesome

The script now:

* Allows specification of a specific release version
  (--release-version x.y.z), looking for the latest release version
  only when no such value is specified.

* Uses a more robust strategy for resolving the release ref (not
  necessarily a tag) which goes along with the desired release
  version: if no suitable tag is found, the given version's JAR
  artifact is obtained from the Maven repository cache (downloading
  it if necessary), and the Implementation-Build is parsed from the
  JAR manifest. This change means that even projects which do not
  tag their releases (e.g., many Fiji components) will still have a
  means to determine the ref of the release.

* More clearly delineates the various tasks into subroutines.

* Has a debug mode (--debug) that emits details of actions taken.

* Removes the obsolete command line flags
  --ask-nexus-for-latest-version and --git-fetch-first.