fix mpm_get_name hook

a string can't be used as the declined value since the hook macro must compare 
retcodes from hooks with that declined value

(thanks, gcc/Joe)

accommodate in ap_show_mpm() since we don't want to return NULL to the caller of
this simple API


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@758406 13f79535-47bb-0310-9956-ffa450edef68