lisa.trace: Restrict the trace_printk formats recognized by lisa.trace.Trace.df_event()

[douglas-raillard-arm]

• No more trace_printk@func@..., only the more basic trace_printk@... will stay. trace_printk@func@... has reliability issues so it would have been deprecated in any case.
• The format string must describe the event schema fully, e.g. "myevent: field1=%i". You cannot play tricks like "%s: field1=%i"
• Literal values are not allowed, e.g. you have to use "myevent: field1=%i" , you cannot do "myevent: field=42" even if you always pass 42.
• You have to have at least one field. Otherwise, there is no variable to interpolate and you will end up with a bputs event instead of bprint, which is not supported anymore.
• Remove support for positional field ("__positional") for meta events such as trace_printk@ or userspace@ events. This feature was not used and complexifies parsers for little gain.