6963027: TEST_BUG: channels and buffer tests need to run in samevm mode

[Alan Bateman]

I've taken a pass over the nio tests so that the jdk_nio2 test target 
(run the buffers and channels tests) now runs in samevm mode. 

The main thing with running tests in this mode is that tests need to 
release resources so that subsequent tests don't fail because all file 
descriptors are used or all socket buffers are exhausted. There are a 
couple of tests that are essentially mini-stress tests and these have 
been known to cause problems for tests that run subsequently. I've 
dialed-down several of these tests. There are a couple of tests that 
I've had to change to run explicitly in their own VM. There are various 
reasons for this, but it's mostly the tests that use memory-mapped 
files. Along the way I fixed a few other random issues but for the most 
part, the changes are easy to review.

The webrev is here:
  http://cr.openjdk.java.net/~alanb/6963027/webrev/

Kelly - I've changed test/Makefile to run this batch in samevm mode. It 
may be that we need to do a bit of re-balancing to even up the run times 
when running several batches concurrently (maybe move the buffer tests 
to jdk_nio1, or jdk_nio3 once it runs in samevm mode).

-Alan.