Synthesize an ifIndex of 0 if the aux object contains an empty string
as indetifier of the AC interface.  If the string refers to an unkown
interface (not part of the table of persistent interfaces), also
synthesize a value of 0 and log a warning.

Always check for a change of the shmem segment at startup
and don't die when the shmem directory doesn't exist.

Subagents use a specific community string to access the sysUpTime
object in the master agent.

Remove excessive comments.

The interface agent is renamed from interfaces.  Both agents now
accept the command-line option "--shmem-dir" to set the directory
where the Snabb shared memory segments reside.

This version works but lacks documentation and needs a thorough
clean-up.