When setting the environment variable EMPER_LOG_FILE=<logfile> EMPER
will write its log messages to <logfile> instead of stderr.
This removes the need for the mutex protecting std::cerr as well as
multiple write calls to flush std:cerr.

To efficiently write log messages to the log file the algorithm uses
three memory 1MiB mapped views into <logfile> to store the log messages.
One buffer is active, one is new, and one is old.
The next buffer ensures that threads can write log messages even if the
active buffer would overflows.
The old buffer allows slower threads to still write safely while everyone
else uses the active buffer.
When a thread changes from the active buffer to the new buffer it
is responsible to renew the current old buffer and changing the semantic
of the buffers:

* active -> old
* next -> active
* old -> next

This buffer scheme allows wait-free logging.
But the approach is NOT sound because delayed thread may still use the
old buffer which gets renewed by the first thread touching the next buffer.
But the likeliness for this situation decreases with bigger sizes of the
buffers.

ATTENTION: Using SCHED_IDLE for the completer may break this likeliness
assumption.

Add new CI test job with mmaped log files.

This contains code cleanups
Suggested-By: Florian Schmaus <flow@cs.fau.de>