MINOR: trace: Be able to chain commands for a source in one line

MINOR: trace: Be able to chain commands for a source in one line

In the configuration file or on the CLI, configuring traces for a specific
source is a bit painful because this must be done in several lines. Thanks
to this patch, it is now possible to fully configure traces for a source in
one line. For instance, the following on the CLI:

  trace h1 sink stderr; trace h1 level developer; trace h1 verbosity complete; trace h1 start now

can now be replaced by:

  trace h1 sink stderr level developer verbosity complete start now

The same is true for the 'trace' directives in the configuration file.