security: Inhibit execution of privilege escalating functions

This patch allows individual dialplan functions to be marked as
'dangerous', to inhibit their execution from external sources.

A 'dangerous' function is one which results in a privilege escalation.
For example, if one were to read the channel variable SHELL(rm -rf /)
Bad Things(TM) could happen; even if the external source has only read
permissions.

Execution from external sources may be enabled by setting
'live_dangerously' to 'yes' in the [options] section of asterisk.conf.
Although doing so is not recommended.

Also, the ABI was changed to something more reasonable, since Asterisk
12 does not yet have a public release.

(closes issue ASTERISK-22905)
Review: http://reviewboard.digium.internal/r/432/
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Merged revisions 403913 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 403917 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 403959 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403960 65c4cc65-6c06-0410-ace0-fbb531ad65f3

configs/asterisk.conf.sample

@@ -83,6 +83,12 @@ documentation_language = en_US	; Set the language you want documentation
 				; gosub - Invoke the stdexten using a gosub as
 				;         documented in extensions.conf.sample.
 				; Default gosub.
+;live_dangerously = no		; Enable the execution of 'dangerous' dialplan
+				; functions from external sources (AMI,
+				; etc.) These functions (such as SHELL) are
+				; considered dangerous because they can allow
+				; privilege escalation.
+				; Default yes, for backward compatability.
 
 ; Changing the following lines may compromise your security.
 ;[files]