--with-jtreg is broken for many use cases

JDK-8179537 was opened with a request to document the brokenness of --with-jtreg. Better to fix it than to document how it's broken.

Now --with-jtreg works like this:
 * --without-jtreg will explicitly disable jtreg.
 * --with-jtreg=<jtreg home> will use the specified directory, and will do sanity checks that it is indeed a jtreg home pointed to, and not a binary.
* When no option is given, we try to locate jtreg using JT_HOME from the environment, and, failing that, by finding the jtreg executable in the path and deriving a JT_HOME from that. If not found, jtreg will be disabled.
* --with-jtreg (with no arguments) will try to locate jtreg automatically as described above, but if it fails, it will abort configure.