In this way Travis script (travis-extract-var.sh) will automatically
pass the var to the docker container. Also, it makes the macro name more
consistent with all the others.
- Split r_log in r_vlog/r_log and make r_assert_log use r_vlog because it
needs to provide a va_list.
- Rename _r_internal_logging to r_log because that's a R_API function
and it needs a good name
- Make R2_CHECKS_LEVEL dependent on 'buildtype' option
- Make R_LOGLVL_WARN the default log level when in debug mode (default)
- introduce ASSERT_STDOUT macro to print assert failures to stdout
- use ASSERT_STDOUT in the Travis env with asan
- use list of callbacks in RLog instead of just one.
