chan_dahdi: Reflect the set software gain in the CLI "dahdi show channel" output.

* Remember the swgain setting from CLI "dahdi set swgain" command so the
CLI "dahdi show channel" output will reflect the current setting.

* Updated CLI "dahdi set hwgain" and "dahdi set swgain" documentation.

(issue ASTERISK-22429)
Reported by: Jaco Kroon
Patches:
      jira_asterisk_22429_v1.8_v2.patch (license #5621) patch uploaded by rmudgett
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Merged revisions 400907 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 400909 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 400911 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@400913 65c4cc65-6c06-0410-ace0-fbb531ad65f3