Change core show help output format.

The CLI "core show help" output leaves something to be desired.
1) The command is truncated to a maximum of 30 characters.
2) The output columns are mirrored from the 31st column.

Current output format:
                   logger mute Toggle logging output to a console
                 logger reload Reopens the log files
                 logger rotate Rotates and reopens the log files
logger set level {DEBUG|NOTICE Enables/Disables a specific logging level for this console
          logger show channels List configured log channels


New format:
logger mute                    -- Toggle logging output to a console
logger reload                  -- Reopens the log files
logger rotate                  -- Rotates and reopens the log files
logger set level {DEBUG|NOTICE|WARNING|ERROR|VERBOSE|DTMF} {on|off} -- Enables/Disables a specific logging level for this console
logger show channels           -- List configured log channels

Review: https://reviewboard.asterisk.org/r/2133/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@374109 65c4cc65-6c06-0410-ace0-fbb531ad65f3