RFR: 8146793: logStream::write re-formats string

[Kim Barrett]

Please review this fix to some problems in logStream::write and the
facilities used to implement it.

1. logStream::write no longer attempt to reformat the already
formatted accumulated output.  [Fixing this led to the discovery of
the issues below.]

2. Fixed Log<>::vwrite test for overflow of the initial buffer, which
didn't allow for the trailing NUL.

3. Fixed Log<>::vwrite to copy the va_list arguments before first
format attempt and use that copy if a second format attempt is
needed, rather than incorrectly reusing the original (now possibly
modified) va_list.

CR:
https://bugs.openjdk.java.net/browse/JDK-8146793

Webrev:
http://cr.openjdk.java.net/~kbarrett/8146793/webrev.00/

Testing:
JPRT
Locally verified printing a "%" to a log stream works, by running
TestPrintGCDetailsVerbose (after fixing it for JDK-8146728).