added option_minmemfree for use in asterisk.conf to specify the amount of...

added option_minmemfree for use in asterisk.conf to specify the amount of minimum free memory prior to accepting calls.  added CLI 'core show sysinfo' to display system information

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@61539 65c4cc65-6c06-0410-ace0-fbb531ad65f3