pmem: add pmem_log_set_function function + UT

[osalyk]

This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.12%. Comparing base (204bdff) to head (ef81768).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6059      +/-   ##
==========================================
- Coverage   70.13%   70.12%   -0.01%     
==========================================
  Files         133      133              
  Lines       19565    19568       +3     
  Branches     3261     3261              
==========================================
+ Hits        13721    13723       +2     
- Misses       5844     5845       +1     

[janekmi]

Reviewed 7 of 7 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @grom72)

---

src/include/libpmem.h line 114 at r1 (raw file):

	/* debug info e.g. write operation dump */
	PMEM_LOG_LEVEL_DEBUG,
};

I assume #6057 will be landed first. So, I do not repeat myself on this bit.

Code quote:

/*
 * Available log levels. Log levels are used in the logging API calls
 * to indicate logging message severity. Log levels are also used
 * to define thresholds for the logging.
 */
enum pmem_log_level {
	/* only basic library info */
	PMEM_LOG_LEVEL_HARK,
	/* an error that causes the library to stop working immediately */
	PMEM_LOG_LEVEL_FATAL,
	/* an error that causes the library to stop working properly */
	PMEM_LOG_LEVEL_ERROR,
	/* an errors that could be handled in the upper level */
	PMEM_LOG_LEVEL_WARNING,
	/* non-massive info mainly related to public API function completions */
	PMEM_LOG_LEVEL_NOTICE,
	/* massive info e.g. every write operation indication */
	PMEM_LOG_LEVEL_INFO,
	/* debug info e.g. write operation dump */
	PMEM_LOG_LEVEL_DEBUG,
};

[grom72]

Reviewed 4 of 7 files at r1, 3 of 3 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @osalyk)