Added support of `--enable-valgrind-regtest` option.

* bootstrap: Added the support of `--enable-valgrind-regtest` and
             `--disable-valgrind-regtest` command line options.
* jamroot:
  * Added the support of `--disable-valgrind` command line option.
  * Only the last one of `--enable-valgrind` and `--disable-valgrind`
    command line options is effective.
  * Removed the use of `option.get` rule because it reconizes
    `--enable-valgrind-regtest` command line option as `--enable-valgrind`
    with the argument `-regtest`.
  * Added the support of `--enable-valgrind-regtest` and
    `--disable-valgrind-regtest` command line options.
  * Added `VALGRIND_REGTEST` global constant.
* valgrind/jamfile: Made `make regtest` skipped if `VALGRIND_REGTEST` is
                    not equal to `yes`.