Logger: Fix handling of absolute paths

This fixes path handling for log files so that an extra / is not
appended to the file path when the path is absolute (begins with /).
This would previously result in different but functionally equivalent
paths in the output of 'logger show channels'.
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Merged revisions 407455 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 407456 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 407458 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@407459 65c4cc65-6c06-0410-ace0-fbb531ad65f3