Fix an existing problem with error handling in piped_log_spawn().

Use new APR apr_proc_create() features to prevent Apache from starting
on Unix* in most cases where a piped log program can be started,
and add log messages for the other situations.  *Other platforms
already failed Apache initialization if a piped log program
couldn't be started.

PR:         15761


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@98607 13f79535-47bb-0310-9956-ffa450edef68