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.
Clang is a bit more strict wrt format-nonliterals:
http://clang.llvm.org/docs/LanguageExtensions.html#format-string-checking
Adding these extra printf attributes also makes gcc able to find more
problems. E.g. this patch uncovers a format issue in udev-builtin-path_id.c
Some parts looked intetional about breaking the format-nonliteral check.
I added some supression for warnings there.
