MINOR: startup: tune.renice.{startup,runtime} allow to change priorities

MINOR: startup: tune.renice.{startup,runtime} allow to change priorities

This commit introduces the tune.renice.startup and tune.renice.runtime
global keywords that allows to change the priority with setpriority().

tune.renice.startup is parsed and applied in the worker or the standalone
process for configuration parsing. If this keyword is used alone, the
nice value is changed to the previous one after configuration parsing.

tune.renice.runtime is applied after configuration parsing, so in the
worker or a standalone process. Combined with tune.renice.startup it
allows to have a different nice value during configuration parsing and
during runtime.

The feature was discussed in github issue #1919.

Example:

   global
        tune.renice.startup 15
        tune.renice.runtime 0