Improve interaction of vectx and tftp

[sjg]

Summary

On slow platforms, it helps to spread the hashing load
over time so that tftp does not timeout.

Also, some .4th files are too big to fit in cache of pkgfs,
so increase cache size and ensure fully populated.

[sjg]

skip reset tries

[stevek]

stand/libsa/pkgfs.c

286

This should be:
if (tf->tf_cachesz == 0)

512

This should be:
if (tf->tf_cachesz > 0) {

[sjg]

fix cachesz checks

[sjg]

fix cachesz checks for sure