implement common error printing function

[jeffpc]

instead of sprinkling the codebase with printf & fprintf calls, provide the one logging function that does the right thing. This may or may not involve logging to syslog as well. Any objections to calling it cmn_err?

Some useful features this function should have:

• take a severity argument (debug, info, warning, error, critical, panic)
• prefix messages with current thread id
• automatically append a \n