MFC r297147, r297148, r297149, r297150, r297151:

MFC r297147, r297148, r297149, r297150, r297151:

  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.

PR:		187094

git-svn-id: svn://svn.freebsd.org/base/stable/10@301056 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f