common: make all calls to pthread_*() uniform

1) Make all calls to pthread_*() uniform:
   - *_lock() - set errno to return value, call LOG() and error out,
   - *_unlock() and *_init() - set errno to return value and call LOG()
2) Change return type of lane_exit() to static void.
common: fix date format in rpm changelog
blk: only one flag allowed at the time

- clear other flags when setting any flag
- add unit tests

Ref: pmem/issues#4
Merge pull request #39 from ldorau/blk-check-return-pthread_mutex_lock

common: make all calls to pthread_*() uniform
Merge pull request #42 from plebioda/fix-date-format

common: fix date format in rpm changelog
Merge pull request #43 from plebioda/btt-clear-error-flag-when-setting-zero-flag

blk: only one flag allowed at the time