[ARGUS] fix select case: ensure read mask cleared first

[slif at bellsouth.net]

the bit field arguments presented to select() must be freshly set
before any bits representing open file descriptors are set.

This patch fixes one case where the read mask was not cleared
before FD bits were set.

I've seen instances where the server would shut down completely
if the select errors out.  The server would log "client done",
and the "ra" client would print a "man" record whose last
field was "SHT".

The server is much more robust when this patch is applied.

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