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 r276412:

Fix loader's ability to read the 10.1 release PowerPC ISOs. There appears to
be some kind of problem with the version of makefs used for these disks.
There may be a better way to handle this problem, so I've set the MFC
timer for a fairly long time period.

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 r276412:

Fix loader's ability to read the 10.1 release PowerPC ISOs. There appears to
be some kind of problem with the version of makefs used for these disks.
There may be a better way to handle this problem, so I've set the MFC
timer for a fairly long time period.