ap_available_mutexes_string and ap_add_available_mutexes_string

ap_available_mutexes_string and ap_add_available_mutexes_string
cannot be data symbols when mod_ssl is built as a loadable module;
using an external string constant in a loadable module is not portable.

Also reorganize file and sem to follow their explicit mechanisms,
and ensured that the explicit mechansims are listed in order of
preference.

This raises a question, would [fcntl|file] be a clearer way of
indiciating what the file/sem methods devolve to?

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@568833 13f79535-47bb-0310-9956-ffa450edef68