manager: Fix could not extend string messages.

When shutting down Asterisk that has an active AMI connection, you get
several "failed to extend from %d to %d" messages because use of the
EVENT_FLAG_SHUTDOWN attempts to add all AMI permission strings to the
event.

* Created MAX_AUTH_PERM_STRING to use when creating stack based struct
ast_str variables used with the authority_to_str() and
user_authority_to_str() functions instead of a variety of magic numbers
that could be too small.

* Added a special check for EVENT_FLAG_SHUTDOWN to authority_to_str() so
it will not attempt to add all permission level strings.

Review: https://reviewboard.asterisk.org/r/4200/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@428570 f38db490-d61c-443f-a65b-d21fe96a405b