Add pcap_open_live() timeout for sock read to Linux backend

[guyharris]

Converted from SourceForge issue 1642214, submitted by egbert_e

The timeout for socket reads (recvfrom()) is not implemented for the Linux backend. This causes problems with nmap which uses libpcap now: when scanning a closed port nmap will hang forever.
The attached trivial patch adds this timeout.

[guyharris]

Submitted by guy_harris

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Any application that thinks that the timeout in pcap_open_live() guarantees that a read will block for no more than the specified amount of time is broken on more platforms than just Linux. In Solaris, for example, the timeout is implemented - but, by design and intent, the timer starts when the first packet arrives (that's the way bufmod's timer works). (I wish BPF had worked that way, so people didn't get used to the idea that you could use the timeout for polling. They might've had to make select()/poll() work with BPF devices sooner than they did....)