introduce 'core show sysinfo' for systems that dont have the Linux-ish sysinfo stuff but do have sysctl.

(closes issue #13433)
Reported by: mvanbaak
Patches:
      2008121300_sysinfosysctl.diff.txt uploaded by mvanbaak (license 7)
	  with two free calls replaced with ast_free based on feedback on reviewboard
Review:
      http://reviewboard.digium.com/r/91/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@164802 65c4cc65-6c06-0410-ace0-fbb531ad65f3