RFR: 8264565: Templatize num_arguments() functions of DCmd subclasses

[Ioi Lam]

We have many version of `num_arguments()` for the `DCmd` subclasses. They all have identical structure. We should templatize them to reduce duplicated code and avoid cut-and-paste errors.

It's still possible to write a customized `num_arguments()` function (although none of the existing cases needs to do that). The rules are described here:

class DCmd : public ResourceObj {
  ...
  // num_arguments() is used by the DCmdFactoryImpl::get_num_arguments() template functions.
  // - For subclasses of DCmdWithParser, it's calculated by DCmdParser::num_arguments().
  // - Other subclasses of DCmd have zero arguments by default. You can change this
  //   by defining your own version of MyDCmd::num_arguments().
  static int num_arguments()        { return 0; }

-------------

Commit messages:
 - removed remaining boilerplate num_arguments() functions
 - DebugOnCmdStartDCmd does not need to subclass from DCmdWithParser
 - 8264565: Templatize num_arguments() functions of DCmd subclasses

Changes: https://git.openjdk.java.net/jdk/pull/3312/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3312&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264565
  Stats: 277 lines in 8 files changed: 27 ins; 241 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3312.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3312/head:pull/3312

PR: https://git.openjdk.java.net/jdk/pull/3312