Make both the loader and kernel use the interface-mtu option if the
dhcp server provides it.  Made up of these (semi-)related changes...

[kernel...] If the dhcp server provides an interface-mtu option, parse
the value and set that mtu on the interface.

[libstand...]

Garbage collect the bswap routines from libstand, use sys/endian.h.

If the dhcp server delivers an interface-mtu option, parse it and store
the value in a new global intf_mtu for use by the application.

[loader...]

If the dhcp server provided an interface-mtu option, transcribe the value
to the boot.netif.mtu env var, which will be picked up by pre-existing code
in nfs_mountroot() and used to configure the interface accordingly.