res_fax: check_modem_rate() returned incorrect rate for V.27

According to the new standard for V.27 and V.32 they are able to transmit
at a bit rate of 4,800 or 9,600.  The check_mode_rate function needed to be
updated to reflect this.  Also, because of this change the default 'minrate'
value was updated to be 4800.

(closes issue ASTERISK-22790)
Reported by: Paolo Compagnini
Patches:
     res_fax.txt uploaded by looserouting (license 6548)
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Merged revisions 405656 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 405693 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@405694 f38db490-d61c-443f-a65b-d21fe96a405b