Handle multiple commands per connection via netconsole

Asterisk would accept multiple NULL-delimited CLI commands via the
netconsole socket, but would occasionally miss a command due to the
command not being completely read into the buffer. This patch ensures
that any partial commands get moved to the front of the read buffer,
appended to, and properly sent.

(closes issue ASTERISK-18308)
Review: https://reviewboard.asterisk.org/r/1876/
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Merged revisions 362536 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 362537 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@362538 65c4cc65-6c06-0410-ace0-fbb531ad65f3