Improve handling of values supplied to FAXOPT(ecm).

Previously, values that began with whitespace were silently treated as 'no',
and all non-'yes' values were also treated as 'no'. Now the supplied value
is specifically checked for a 'yes' or 'no' (or equivalent) value, after skipping
leading whitespace. If the value is not valid, then a warning message is generated.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@252709 65c4cc65-6c06-0410-ace0-fbb531ad65f3