net-test: packetdrill: allow 32-bit system call parameters to be s32 or u32

To allow hexadecimal integers with the most significant bit set to be
passed as system call parameters, we need to broaden the set of values
acceptable to get_s32() to include u32.

If this proves confusing, then at some point in the future we could
rename this to be get_32() or something.

Change-Id: I416f80f8c135306c6b64de8b700186d0b9a005c7