Use log "level" instead of "priority"

Use log "level" instead of "priority"

The term "priority" is misleading because higher levels have lower
priority. "Level" is clearer and shorter.

This commit touches only the textual descriptions, not function and variable
names themselves. "Priority" is used in various command-line switches and
protocol constants, so completly getting rid of "priority" is hard.

I also left "priority" in various places where the clarity suffered
when it was removed.