[util/subprocess] Log subprocess start level at debug2

The debug1 log level consisted entirely of spam about command execution;
if users are interested in seeing the executed commands they generally
want to see the output as well so logging start and finish messages at
different levels provided zero value. This commit makes all subprocess
debug messages log at the same level.
Merge pull request #229 from adrienthebo/maint/master/subprocess-start-loglevel

[util/subprocess] Log subprocess start level at debug2