mpm_winnt: Tweak the names of the variables in child.c which are used to

mpm_winnt: Tweak the names of the variables in child.c which are used to
represent a queue of the completion contexts.

Starting from r1801655, the "queue" isn't really a queue, as all the
access happens with a LIFO order. So, instead of that, call it a "pool
of completion contexts", adjust names of all relevant variables and
tweak the comments.

This patch changes
  - qlock to ctxpool_lock,
  - qhead to ctxpool_head, and
  - qwait_event to ctxpool_wait_event.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1801748 13f79535-47bb-0310-9956-ffa450edef68