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

MFC r276079, r276087:

Add a divisor parameter to twiddle() so that callers can request that
output only happen on every Nth call.

Add a new loader(8) variable, twiddle_divisor, allowing control over the
output frequency of the "twiddle" IO progress indicator.

MFC r266878, r266879: Add support for snprintf() to libstand.

MFC r276079, r276087:

Add a divisor parameter to twiddle() so that callers can request that
output only happen on every Nth call.

Add a new loader(8) variable, twiddle_divisor, allowing control over the
output frequency of the "twiddle" IO progress indicator.

MFC r266878, r266879: Add support for snprintf() to libstand.