make STP_OOB_DATA treatment more stringent

make STP_OOB_DATA treatment more stringent

Since all STP_OOB_DATA originally comes from _stp_vlog() in
runtime/linux/io.c, we can be sure that all warnings will be prefixed
by "WARNING: " and all errors by "ERROR: ", as defined by WARN_STRING
ERR_STRING in io.c.

Therefore, we can be more stringent in control.c and mainloop.c in the
way we identify warnings and errors. Additionally, warnings can instead
use the warn() macro and errors the err() macro so that they benefit
from coloring.

The main effect of this patch is that all calls to _stp_warn() and
_stp_error() will be colored upon reception by stapio.