CLEANUP: stats: rename the stats state values an mark the old ones deprecated

CLEANUP: stats: rename the stats state values an mark the old ones deprecated

The STAT_ST_* values have been abused by virtually every applet and CLI
keyword handler, and this must not continue as it's a source of bugs and
of overly complicated code.

This patch renames the states to STAT_STATE_*, and keeps the previous
enum while marking each entry as deprecated. This should be sufficient to
catch out-of-tree code that might rely on them and to let them know what
to do with that.