when parsing a text configuration option, ensure that the buffer on the stack is...

when parsing a text configuration option, ensure that the buffer on the stack is actually large enough to hold the legal values of that option, and also ensure that sscanf() knows to stop parsing if it would overrun the buffer (without these changes, specifying "buffers=...,immediate" would overflow the buffer on the stack, and could not have worked as expected)

git-svn-id: http://svn.digium.com/svn/asterisk/branches/1.4@147681 614ede4d-c843-0410-af14-a771ab80d22e