tools: netmap: pkt-gen: check packet length against interface MTU

Validate the value of the -l argument (packet length) against the MTU of the netmap port.
In case the netmap port does not refer to a physical interface (e.g. VALE port or pipe), then
the netmap buffer size is used as MTU.
This change also sets a better default value for the -M option, so that pkt-gen uses
the largest possible fragments in case of multi-slot packets.

Differential Revision: https://reviews.freebsd.org/D18436