Pls review (S) 7126185: Clean up lasterror handling, add os::get_last_error()

[Paul Hohensee]

Webrev here

http://cr.openjdk.java.net/~phh/7126185.00/

Delete getLastErrorString() from os_windows.cpp and replace its uses by
calls to os::lasterror().  getLastErrorString() was functionally 
identical to
os::lasterror().

Add os::get_last_error() to get the last error number.  Used only by closed
source code right now, but logically belongs in the open.

Use the correct DWORD type for the result of GetLastError().

Thanks,

Paul