use stderr for error messages and for ncurses ui

[JCoMcL]

I tried writing a shell script which parses the output of radeontop -d - and found that I had to filter out the first two lines

Unknown Radeon card. <= R500 won't work, new cards might.
Dumping to -, line limit 1.

Printing metadata like this to stderr for humans to see while and reserving stdout for the actual data makes piping a lot easier. Despite the name, stderr isn't just errors, it's generally for anything intended for humans and not meaningful in shell scripts. That also includes the ncurses UI, which typically uses stderr.
These changes should make no difference in normal cases but be a lot more predictable in specific cases.