libflux/flog: Support output to stderr

[chu11]

Make flux handle optional in logging functions. If flux handle
is NULL, output to stderr.

Add unit tests appropriately.

Fixes #1191

[chu11]

I think primary review is on format of output:

return fprintf (stderr, "%s: %s\n", lstr, buf);

No colon? Upper case priority level string? I elected on colon and lower case.

[garlick]

Looks good! Don't forget a mention in flux_log(3).

[coveralls]

Changes Unknown when pulling 7fd9a3b on chu11:issue1191 into ** on flux-framework:master**.

[chu11]

Hmm, I had one build failure

PASS: t0001-basic.t 53 - instance can stop cleanly with subscribers (#1025)
ERROR: t0001-basic.t - missing test plan
ERROR: t0001-basic.t - exited with status 1

Which suggests my test seg-faulted or something like that. hmmm.

[garlick]

No colon? Upper case priority level string? I elected on colon and lower case.

Sounds ok to me.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@06a2ec8). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #1192   +/-   ##
=========================================
  Coverage          ?   77.81%           
=========================================
  Files             ?      158           
  Lines             ?    29288           
  Branches          ?        0           
=========================================
  Hits              ?    22789           
  Misses            ?     6499           
  Partials          ?        0

Impacted Files	Coverage Δ
src/common/libflux/flog.c	93.49% <100%> (ø)

[coveralls]

Changes Unknown when pulling 670fbaa on chu11:issue1191 into ** on flux-framework:master**.

[chu11]

pushed manpage updates & pushed fix for issue I listed above (failed chain lint was the issue).

first run through travis hit #1169 and some write errors, restarting

[garlick]

Thanks! Looks good.