asterisk.c: Add option to restrict shell access from remote consoles.

UserNote: A new asterisk.conf option 'disable_remote_console_shell' has
been added that, when set, will prevent remote consoles from executing
shell commands using the '!' prefix.

Resolves: #GHSA-c7p6-7mvq-8jq2

configs/samples/asterisk.conf.sample

@@ -138,6 +138,9 @@ documentation_language = en_US	; Set the language you want documentation
                 ;   cpp_map_name_id: Use C++ Maps to index both
                 ;                    channel name and channel uniqueid.
                 ; See http://s.asterisk.net/dc679ec3 for more information.
+;disable_remote_console_shell = no; Prevent remote console CLI sessions
+                ; from executing shell commands with the '!' prefix.
+                ; Default: no
 
 ; Changing the following lines may compromise your security.
 ;[files]

configs/samples/cli_permissions.conf.sample

@@ -19,6 +19,11 @@
 ; deny = <command name> | all		; disallow the user to run 'command' |
 ;					; disallow the user to run 'all' commands.
 ;
+; NOTE: This file can't be used to restict the use of the '!' prefix
+; for running shell commands from the CLI. You can however disable the
+; use of the shell commands in remote consoles altogether by setting
+; the 'disable_remote_console_shell' parameter in asterisk.conf to 'yes'.
+;
 
 [general]