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

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add support to send the string 'PXEClient' as the Vendor class
identifier to the DHCP server. Now you can check for this string
in your dhcp configuration to decide whether you will hand out a
lease to the client or not.

This commit was generated by cvs2svn to compensate for changes in r38451,
which included commits to RCS files with non-trunk default branches.

This is libstand; a support library for standalone executables (eg. bootstrap
modules).
Obtained from: NetBSD, with some architectural changes and many additions.