Evaluate nested If/ElseIf/Else config sections

Evaluate nested If/ElseIf/Else config sections

It has been reported multiple times that nested
If/ElseIf/Else sections are not evaluated but
silently ignored.

This patch adds a simple recursion to the ap_if_walk
logic in order to allow arbitrary nested configs.
The overhead seems negligible compared to the actual
version of the ap_if_walk, but more expert feedback
is surely needed since this code gets called for every
HTTP request.

Tests are going to be added to t/apache/if_sections.t

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1792589 13f79535-47bb-0310-9956-ffa450edef68