CA communicates diagnostics three ways - why is that

[Jeff Hill]

CA currently communicates diagnostics three ways:
1) exception callback
2) CA printf callback
3) errlogPrintf

The 2nd way arose because its not easy to force all diagnostic messages into an exception (the exception
callback needs a printf format and varargs).

The third way arose because there are certain situations where CA has a lock applied and it cant easily be released
before calling the printf handler. CA currently tries to never hold a lock when calling a callback.

See also #326

Original Mantis Bug: mantis-238
    http://www.aps.anl.gov/epics/mantis/view_bug_page.php?f_id=238