RFR (10) (S) 8175817: Clean up Solaris signal code: SIGUSR2, SIGasync, SIGJVM1/2

[David Holmes]

Bug: https://bugs.openjdk.java.net/browse/JDK-8175817

webrev: http://cr.openjdk.java.net/~dholmes/8175817/webrev/

On Solaris SIGasync represents what we call the "SR signal" on other 
platforms (the event-based suspend/resume signal), and it is hardwired 
to SIGJVM2 (a dedicated JVM signal on Solaris 10+).

As this signal is hard-wired and has not been user-definable at runtime 
for a very long time, we can do away with the SIGasync field and its 
setter and getter, and simply define ASYNC_SIGNAL as SIGJVM2 (not 
SIGUSR2 as the code currently suggests!). Once this is a build time 
constant it simplifies some other code.

We can also stop trying to account for SIGJVM1/2 not existing as they 
must exist on the supported Solaris platforms.

We can also stop trying to deal with an older libjsig (the logic for 
which has a bug anyway: used <= instead of < in the version check).

Overall just a bit of simplification and cleanup. No functional changes.

Testing: JPRT

Thanks,
David