res_fax: Warn that minrate=2400 is not valid for V.27 instead of failing load.

Change minrate from 2400 to 4800 on config reload in response to changes from
ASTERISK-22790 only.  Any config with minrate of 2400 that would fail before
r405693 will still fail.

Comment out many settings in res_fax.conf.sample. The defaults are set in
res_fax.c, so setting the same value in sample config does nothing but make
the sample config more fragile.

(closes issue ASTERISK-23231)
Reported by: David Brillert
Review: https://reviewboard.asterisk.org/r/3261/
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Merged revisions 409052 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 409053 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@409054 f38db490-d61c-443f-a65b-d21fe96a405b