RFR: JDK-8180281 --with-jtreg is broken for many use cases

[Magnus Ihse Bursie]

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.

Bug: https://bugs.openjdk.java.net/browse/JDK-8180281
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8180281-with-jtreg-is-broken/webrev.01

/Magnus