automatically bump HG_LOG_SUBSYS when HG_LOG_LEVEL is set

[carns]

Is your feature request related to a problem? Please describe.

See mochi-hpc/mochi-thallium#7 (comment). The log subsys is not implicitly changed in response to the log level when set via env variable, but it is when set by library API, if I understand correctly.

Describe the solution you'd like

It would probably be better if they were consistent, so that users who want to see non-fatal error messages can get there with one environment variable.

Describe alternatives you've considered

We are going to address this in Margo via mochi-hpc/mochi-margo#233 (we would prefer a higher default log level for our use case anyway so we may as well take explicit control over both the log level and log subsys for our purposes), just noting that maybe the inconsistency between the env variables and and programmatic API might be a good thing to fix independently at the Mercury level for non-Margo users.

[soumagne]

I fixed that just now, setting HG_LOG_LEVEL without setting HG_LOG_SUBSYS will be equivalent to setting HG_LOG_SUBSYS=hg_all (meaning all subsys enabled)