RFR(s): 8148219: Add decorator hostname to UL

[Robbin Ehn]

Hi, please review.

This adds a new decorator for hostname to UL, with minor changes to 
os::get_host_name and UL init.

JBS: https://bugs.openjdk.java.net/browse/JDK-8148219
Webrev: http://cr.openjdk.java.net/~mlarsson/rehn/8148219/

Manual tested and verified no change to hs_err_pid (uses 
os::get_host_name when fastdebug build) and that UL prints hostname.

Thanks!

/Robbin