Support systemd socket activation

Check if systemd passed us a socket when starting obmc-console before
creating the socket ourselves. This will support use cases where we
don't need obmc-console un

Support systemd socket activation

Check if systemd passed us a socket when starting obmc-console before
creating the socket ourselves. This will support use cases where we
don't need obmc-console until a SoL is activated by the user.

Tested: Verified that existing unit file installation is unchanged -
statically enabled obmc-console@.service instances will be started.

Then disabled obmc-console@ttyS2.service and enabled
obmc-console@ttyS2.socket (with a drop-in change to override
ListenStream to "obmc-console") and verifed that activating Redfish and
SSH SoL would start obmc-console@ttyS2.service and console redirection
worked.

Change-Id: I42e96af46a5893145abf27761e97fd4f1b73719d
Signed-off-by: Jonathan Doman <jonathan.doman@intel.com>

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