TS-1834: logstats fails with a missing log directory

traffic_logstats fails when the log directory is missing or not
configured. But really, we only need the log directory in incremental
mode.
# This is a combination of 2 commits.
# The first commit's message is:

Add varargs logging macros

# This is the 2nd commit message:

Add const to Diags members
TS-1850: improved SSL error reporting

Many SSL certificate management errors don't report the full OpenSSL
error stack, making the error difficult to diagnose. We should log
the full error stack in those cases.

Add varargs logging macros to Diags.h.  Add const to Diags members
where possible.  Improve SSL certificate error reporting