Scenario for #153: set IPv4 ECN field

This doesn't seem to work; the ECN field doesn't get set.
In linc_us4_packet:set_field, values are always whole bytes

For VLAN PCP, IP DSCP and IP ECN, expect a whole byte, and extract the
bits we care about.
Extend scenario for #153 to also set IPv4 DSCP field

The way it works now, the lower 6 bits of the value in the Set-Field
action are used as the upper 6 bits of the byte in the IPv4 header that
contains both DSCP and ECN.  It's not completely obvious to me that this
is what the spec says, so this scenario might be useful as a reference
in the future.
Merge pull request #154 from legoscia/dscp-ecn-scenario

Fix Set-Field for fields with non-integral number of bytes