MFC 305588

pxeboot: Add nfs.read_size tunable.

Increase this tunable improves kernel loading speed.

Submitted by: Jun Su <junsu microsoft com>
Reviewed by: rpokala, wblock (previous version)
Obtained from: DragonFlyBSD
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D7756

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 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.