Properly detect and report invalid maxrate and maxrate values in the FAXOPT dialplan function.  Also make fax_rate_str_to_int() return an unsigned int and return 0 instead of -1 in the event of an error.

FAX-202


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@274243 65c4cc65-6c06-0410-ace0-fbb531ad65f3