net-test: packetdrill: support inet_addr() as an expression

packetdrill already supported an inet_addr() as a part of a sockaddr
expression (since that's how strace represents the sockaddr argument
to connect(), for example). But now we also want to allow a
stand-alone inet_addr() call, to allow the specification of IP
addresses for other contexts.

Change-Id: Ia69751030eff399e6b0d9cbb4a15530f0b721278