mpm_winnt: Avoid using TerminateThread() in case the shutdown routine

mpm_winnt: Avoid using TerminateThread() in case the shutdown routine
hits a timeout while waiting for the worker threads to exit.

Using TerminateThread() can have dangerous consequences such as deadlocks —
say, if the the thread is terminated while holding a lock or a heap lock
in the middle of HeapAlloc(), as these locks would not be released.
Or it can corrupt the application state and cause a crash.

(See https://msdn.microsoft.com/en-us/library/windows/desktop/ms686717)

Rework the code to call TerminateProcess() in the described circumstances
and leave the cleanup to the operating system.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1801636 13f79535-47bb-0310-9956-ffa450edef68