Update public interface for logging

Users just call one of five routines:

cvmix_log_verbose
cvmix_log_namelist
cvmix_log_diagnostic
cvmix_log_warning
cmvix_log_error

And a log entry with the proper StatusCode is added (assuming StatusCode is no
smaller than MinMessageLevel). Printing (and erasing) are still public, and now
cvmix_log_drv is a lot more readable. I also cleaned up some of the logging in
the main cvmix_driver.F90 file.