module load priority

This patch adds the option to give a module a load priority. The value represents the order in which a module's load() function is initialized.  The lower the value, the higher the priority.  The value is only checked if the AST_MODFLAG_LOAD_ORDER flag is set.  If the AST_MODFLAG_LOAD_ORDER flag is not set, the value will never be read and the module will be given the lowest possible priority
on load.  Since some modules are reliant on a timing interface, the timing modules have been given a high load priorty.

(closes issue #15191)
Reported by: alecdavis
Tested by: dvossel

Review: https://reviewboard.asterisk.org/r/262/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@199743 65c4cc65-6c06-0410-ace0-fbb531ad65f3