Allow restart of httpd to occur even with syntax errors in the config

Allow restart of httpd to occur even with syntax errors in the config
file.  (Out-of-date DSOs with bad MMNs will still be fatal unfortunately.)

Add return parameter to ap_process_config_tree - OK on success, !OK on
syntax error.  We will no longer call exit() from ap_process_config_tree.
The caller must exit if there is an error (makes sense anyway).  This allows
the initial start-up code to delay the exit until trying to let the
signal_server optional function execute first.

(The chances are that the syntax error isn't in the PidFile directive.  If
that happens, we'll try the default one.  Oh, well.)

PR: 16813

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@98693 13f79535-47bb-0310-9956-ffa450edef68