RFR: 8226816: add UserHandler calls to event log

[Baesken, Matthias]

Hello, please review  the following  small  patch .
It adds  event logging to  the UserHandler  (user signal handler) calls .

(additionally  it adds   a function   os::win32::get_signal_name
to get  signal names for signal numbers ; this is similar to what we already had for posix ).



Bug/webrev :

https://bugs.openjdk.java.net/browse/JDK-8226816

http://cr.openjdk.java.net/~mbaesken/webrevs/8226816.0/

Thanks, Matthias